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Skeletal and muscular adaptations to a subterranean environment of Microtus oregoni serpens (Mammalia… Shaw, Dorothy Anne 1947

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(2D Skeletal and Muscular Adaptations to a Subterranean Environment of Microtus oregoni serpens A Thesis Submitted in Partial Fulfilment of the Requirements for the Degree of MASTER OF ARTS " in the Department of ZOOLOGY The University of British Columbia April, 1947. (Mammalia - Rodent ia) Dorothy Anne' Shaw Contents ' Page I n t r o d u c t i o n • 1 E x t e r n a l F e a t u r e s (a) of M i c r o t u s o r e g o n i s e r p e n s 3 (b) c o m p a r a t i v e • 5 S k e l e t a l S y s t em - c o m p a r a t i v e 8 (a) V e r t e b r a l column 8 (b) R i b s and Sternum 12 (c) S k u l l s 13 (d) P e c t o r a l G i r d l e and Limb 17 (e) P e l v i c G i r d l e and Limb 23 M u s c u l a t u r e - c o m p a r a t i v e 28 (a) B r a n c h i o m e r i c 28 (b) Ivfyotomic 35 A. o f body ( i ) d o r s a l d i v i s i o n 35 ( i i ) v e n t r a l d i v i s i o n . . 37 B. A p p e n d i c u l a r . 42 ( i ) p e c t o r a l g i r d l e and f o r e -l i m b - e x t e n s o r s f l e x o r s 42 ( i i ) p e l v i c g i r d l e and h i n d l i m b - e x t e n s o r s f l e x o r s • 52 D i s c u s s i o n 61 I l l u s t r a t i o n s B i b l i o g r a p h y . Acknowledgement Throughout the course of this study I have been indebted te Dr. Ian McTaggart-Cowan for his continued interest and guidance and for the collection of the specimens used. I would also l i k e to thank Dr. W. A. Clemens, head of the Department of Zoology and the other mem-b e r s of the department for their assistance* Abstract. » . . -This study was undertaken primarily to discover the skeletal and muscular anatomy of a small species of f i e l d mouse, Microtus oregoni serpens Miriam, which inhabits the Puget Sound area of British Columbia and Washing-ton. This species is largely subterranean in i t s habits and in an attempt to discern the degree of specialization for i t s habitat comparison was made between serpens and Microtus townsendi townsendi,(Bacfaman) and Microtus  longicauda vellerosft^lAHen) species which burrow to a limited degree only. A detailed comparison of the external features, skeletal and muscular anatomy of the three species was made and a juvenile specimen of Microtus richardsoni richardsoni,! De Kay)was used for some comparisons. Certain parts of Microtus oregoni serpens were found to be strongly modified in a direction which seems to better adapt the animal to i t s subterranean mode of l i f e . The external features which are of adaptive significance are the soft plush-like pelage, the short t a i l and the arrangement of the vibrissas the longest ones being furthest from the snout,rather than scattered. The eyes are only about one-half the size of those of the larger species and associated with them i s an interesting modification of the orbicularis oculi muscle whereby i t is strongly f&ad&J&ed doubtless as ah aid in pre-venting dirt from entering the eyes. Microtus oregoni serpens does not have conspicuously enlarged or elongated fore-feet but among the modifications for digging which do occur are the shortening of the limbs and the inclusion of a larger part of them within the body skin. Almost the entire musculature of the anterior seg-ment of the body is more stongly developed in serpens including the -'-=-muscles of the shoulder, chest, and fore-limbs and to some extent the masticatory musculature and the musculature of the spinal column. The muscles attached to the pectoral girdle show the most marked modifications they are almost universally better-developed in serpens than in townsendi or longicauda. Correlated with a greater development of the pectoralis muscles i s the development of a Heel on the manubrium stem! and the sternebrae in serpens, providing a larger area for muscle attachments. Microtus oregoni serpens seems to represent one of the earliest stages of adaptive specialization of a mammal for subterranean l i f e extern ally approaching the condition found in specialized burrowers such as the mole but having no radical skeletal modifications. 1. Introduction. Microtus oregoni serpens Merriam, i s a small species of f i e l d mouse inhabiting the Puget Sound region of British Columbia and Washington. It i s largely subterranean i n i t s habits, excavating a maze of small burrows f a i r l y close to the surface. This study has been concerned primarily with the skeletal and muscular anatomy of this mouse. In an attempt to discern, the degree .of specialization for i t s subterranean habitat, comparison has been made between serpens and Microtus townsendi townsendi (Bachman), a species l i v i n g a less fossorial existence, and Microtus longicaudus yeller- osus (Allen), a species which burrows to a very limited degree only. A juvenile specimen of Microtus richardsoni richardsoni (De Kay) was used for certain comparisons and a description of the external features of one specimen Microtus pennsylvanicus drummondi (Audubon and Bachman) is included. The muscles are grouped according to a plan described by Howell (1936) and modified by H i l l ( 1 9 3 7 ) . In general the innervation of the muscles was not investigated in the present study. For comparison of the muscles the terms used are largely those of Howell ( 1 9 S 6 ). The bones were described according to the plan used by H i l l (1937) and comparisons are given where variations occur. The muscle and bone dissections of serpens were done three times and many of the muscles were checked i n a fourth specimen; the muscles and bones of townsendi and longicauda were dissected i n one specimen only of each species but as very few variations occur between the two latter species other than those of size and t a i l structure the two dissections served, in some measure, to check each other. The specimen of richardsoni was a juvenile and, in consequence, was of but limited use for comparative purposes. An attempt has been made to correlate the muscle and skeletal variations and to relate them to functions of the muscles and, £¥5'pos8ible, also, to the habits of the species. 3. External Features Microtus oregoni serpens i s a small-sized rodent with an elongate, slightly fusiform body. The head i s relatively small but i t s general shape i s also elongate. A slight cervical constriction i s present. The skin i s tight a l l over the body and the pelage i s fine, soft, and thick with a velvety texture not found in townsendi or longicaudmg. Scattered guard hairs occur over the surface of the body. The t a c t i l e vibrlssae vary in length from very long on the outer margins to short in the region of the rhinarium but the majority of the bristles are intermediate between these two extremes. There are no dis-tinct interramal and carpal vibrissas but there are a few bristles present on the radial side of the f i r s t digits, though never enough to be termed a tuft. Similar scattered bristles occur on the hind feet. Eyelashes are absent. There are four pairs of mammae, two pectoral and two inguinal. The two halves of the upper l i p meet below the rhinarium to form a very short pbiltrum 0 The lobes of the lower l i p fuse but the l i p is free for some distance. The nearly-naked rhinarium i s triangular in shape with *" the apex directed yentrally. The external nares open lat e r a l l y and have their margins wrinkled and s c r o l l - l i k e . An internarial groove is present. The small, weak-looking eyes bear vestigial nictitating membranes and are provided with small lachrymal glands. The ears average 12 mm-:, in the long axis and are oval In shape. The anterior portion of the helix i s folded and the antinelix forms a ridge margining the conchal cavity. The external ear is almost completely naked with a few scattered hairs on i t s surface. /(^ere^e-af-^pe-c*fne^.) The t a i l i s very much shorter than the body having an average (°f 5) measurement of 'S mm;;, for specimens with an average body length of /s/ mmfv. The fore and hind limbs are slender, the proximal parts of both limb8 are included in the body skin down to the level of the distal ends of the humerus and femur. The front feet are small and delicate and possess spade-shaped claws which are approximately 2 mm;;, in length. The pollex is separated from the other digits, is not opposable, and lacks a nail ending instead with a horny t ip. A l l the digits are short and the metacarpal elements are about equal in length to the digits, thus the manus as a whole is fairly long and slender. The volar surface is covered with a horny rugose skin bearing five plantar tubercles. The two carpal pads are large, the thenar pad covers most of the falciform bone. There is no distinct "heel" dn the fore-foot. The hind feet are less specialized. The second, third and fourth digits are longer than the other two, the fifth digit being the smallest. The claws are not very long and are concave resembling an overgrown human finger n a i l . Microtines are plantigrade and the feet are not greatly elongated. The plantar surface bears a series of five warty tubercles consisting of plantar, apical and interdigital pads and a metatarsal pad which is represented by the thenar pad at the base of the hallux. 5. Comparison of the External Features. The relative proportions of certain external features of serpens, townsendii, longicaudate, and drummondi are shown in the following table. Name N Total Length Length of T a i l Length of Head Length of ] Hind Limb I ength of 'ore-limb Serpens 18 131 mm. 32 mm. 32 mm. 18 mm. 14.5 mm. townsendii 16 176 ram. 55 mm. 35 mm. 26 mm. 18 mm. longicauda 20 185 mm. 62 mm. ' 35.mm. 23 mm. 20 mm. drummondi 14 155 mm. 40 mm. 44 mm. 31 mm. 15 mm. The measurements of total length, length of t a i l , and of hind limb are the average of measurements taken from .a large number of specimens. Microtus serpens and drummondi are alike in that they possess rather slender feet as compared to the relatively stout seet of townsendii, longicauda and richardsoni. The pollex i s without a n a i l i n a l l cases, in serpens and townsendii i t ends in a hard t i p and the other species lack a true pollex the latter being represented by a slight elevation. None of the species possesses an opposable pollex. As regards claw structure, none of the species examined i s provided with conspicuously long or heavy claws of the type found on the fore-feet of many burrowing animals. However, those of serpens and drummondi are relatively much more slender than are those of townsendi and longicauda. The volar surfaces of the feet are harder and rougher i n serpens and townsendi than i n the other species in which i t appears to be relatively smooth. Apical pads are present i n a l l species as also are interdigital pads between digits three and four and carpal pads. The hind feet are less specialized i n a l l five species. The second, third, and fourth digits are always the longest. The claws are short on a l l species except longicauda in which they are long, sharp and decidedly curved. The foot Is definitely lengthened in a l l of the species. The plantar surfaces are always bare but those of serpens and longicauda are less scutellate being hard and roughened. The plantar pads are small in serpens and drummondi and longer in the other species. The hind feet of serpens and townsendi are most alike, longicauda has an extra plantar pad. Considerable variation of length, general shape, scutellation and hairiness is found in the tails of the four species. The tails of serpens  drummondi are similar in length but not in texture the latter being cover* ed withe** distinct scales similar to those found on the t a i l of townsendi. Longicauda has the longest and most bushy tai l in contrast to serpens which has the shortest. The latter feature may be advantageous to an animal which lives underground. Considerable variation is exhibited with regard to looseness of skin and amount of pelage. Serpens and richardsoni have tight skins, longicauda has a slightly looser and townsendi a s t i l l looser skin which is s t i l l , however, much tighter than that of drummondi. The skins seem to be looser on the terrestial forms. The pelage of serpens is fine, soft, and velvety, in the terres-t i a l forms i t i s heavier, thicker, and longer although that of drummondi is fairly thin. Richardsoni has the most dense hair, i t is very heavy on the dorsal side and finer on the underside. The tactile vibrissas are distincVin serpens, the majority of the hairs being of medium length with the longest ones laterally placed. Drummondi has fewer and shorter bristles and townsendi has a much larger percentage of long hairs which are scattered. In richardsoni and longicauda the majority of the vibrissas are long and the longest ones 7. seem to be more medially placed. Scattered guard hairs occur in a l l of the species.. Eye-lashes are absent in serpens and townsendi but there are a few present in richardsoni, drummondi, and longicauda along the dorsal mar-gin of the eye. The structure of the nares and rhinarium in serpens seems to present some adaptive specialization. In this species the nostrils have crenulated margins while those of townsendi and longicauda bear three or four deep grooves on the lateral surfaces, and those of drummondi and richardsoni are smooth. There is a deep internarial groove in serpens while this is indifferently developed in the other species. Serpens has the smallest eyes while those of the other species are larger and in townsendi and longicauda are twice the size of those of serpens. The ears of serpens appear to be of intermediate size, measuring the same length as those of the much larger longicauda. Those of drummondi and richardsoni are similar in size but larger. Considered in percent of total length lbe ears are seen to be of similar size in a l l of the species. The pinnae are oval in serpens and longicauda and more rounded in the .other species. The anterior part of the helix i s folded in each case but this is most noticeable in serpens where i t may have an adaptive value for subterranean l i f e . 8. Skeletal system Vertebral Column-The vertebral column consists of seven cervical, thirteen thoracic, seven lumbar, and five sacral vertebrae. The number of caudal vertebrae varies from seventeen to nineteen. The angle between the cervical and thoracic segments in serpens is approximately 100° and the thoracic seg-ment is strongly arched; the latter character being much more marked in serpens than i t - i s in townsendi, longicauda, or richardsoni. This angle was measured in townsendi as approximately 80°. Cervical Vertebrae-The cervical vertebrae represent approximately 12$% of the pre-sacral length and approximately 14% of the thoracolumbar length. In pre-eo-pared specimens they appear to be dove-tailed together and almost ossified. Atlas- v The atlas is oval in shape with the transverse axis the longer, the ventral arch being shorter. The wings are large in proportion to the size of the body of the atlas and are roughened as tubercular*. The cranial articulating surfaces are large and shallow. No dorsal spine is present in any of the species. There is a difference in size between the atlas of serpens and townsendi or longicauda the latter being slightly larger than would be expected on the basis of relative size. There i s a general tendency to flattening in the cervical verte-brae of a l l the species but this is least evident in longicauda. Axis- (Epistropheus) The dens of the axis is fairly long and ovoid;. The cranial 9. articulating; facets are triangular and convex. The spine is at a 30° angle with the vertical and is relatively large, i . e . , compared to the size of the axis. The vertebral foramen is large and almost circular and the cen-trum slightly concave dorsally. The transverse process iswll-developed and not merely represented by a v-shaped arch such as H i l l describes for Thompmjs (1937). In longicauda and townsendi the transverse processes are less well-developed and are better described as elevations than as true processes. Vertebrae 3 - 7-The other cervical vertebrae are much alike but become progres-sively more slender posteriorly. The bodies of the vertebrae are elongate in the transverse axis and shorter in the other two axes but the foramen is always more circular than ovoid so the degree of elongation cannot be very great. The cranial articulating surfaces are concave and the caudal surfaces are convex. The spinous processes should probably be considered to be absent. The transverse processes are directed laterally throughout. The dorsal and ventral tubercles on the transverse processes are poorly differentiated. There are no true differences between the cervical vertebrae of serpens and townsendi. No spines are found on the dorsal arches in either species. The transverse processes are never branched. In longicauda the vertebrae are more flattened in the vertebral axis making the transverse foramen more ££roid than i t i 3 in serpens or townsendi. The articular processes are more definitely distinguishable in longicauda, i . e . , they project craniad to a greater degree. There is also a slight spine present on the dorsal arch of the fifth, sixth, and seventh vertebrae and the bodies of vertebrae are more steroid in this species. 10. Thoracic Vertebrae* . The thoracolumbar region contains twenty vertebrae, thirteen thoracics and seven lumbars. The first two are very similar to the cervicals and the others become progressively larger, the most caudal ones being almost twice as long as the cervicals. The vertebral foramina be-come smaller caudally while the spinous processes increase in height in the same direction progressively. The spines a l l incline slightlycaudad with the angle of inclination becoming greater in the more caudal vertebrae. Both dorsal and ventral elements of the transverse processes are developed, the former serve for muscle attachment and the latter articu-late with the ribs. The dorsal elements may be indistinctly divided into metapophyses and diapophyses in the eleventh thoracic segment. The articu-lating surfaces are similar to those found in the cervical vertebrae. In a l l three species the thoracic vertebrae show the same general tendencies of the foramina to decrease in size in the more caudal vertebrae and of the spines to become more elongated and caudally directed. Dorsal and ventral elements of the transverse processes are found in a l l of the species. The same tendency toward flattening, found in the cervical vertebrae of longicauda, is found here. The variations found are less specific in the thoracic vertebrae than in the cervicals but the differ-ences are not great in either case. Lumbar Vertebrae-The lumbar vertebrae in serpens are stouter and nearly twice as long as the thoracics. An inconspicuous medial ridge is present on each vertebra. The vertebral foramina become decreasingly smaller toward the end of the lumbar series and the spinous processes become very slightly increased in height posteriorly. The articulating surfaces are like those. 11 of the thoracics. Diapophyses are present on a l l of the lumbars although they are poorly developed in the f i r s t few. This series i s similar in each of the species. The mammillary processes are more distinct in townsendi and longicauda. As few muscle variations occur in this area few variations in the vertebrae are l i k e l y to have occurred. Sacral Vertebrae* Five sacral vertebrae were found in a l l specimens of serpens dissected and according to H i l l (1937) five i s the usual number in Micro-tines. The vertebrae, but not the spinous processes, appear to be fused. The f i r s t sacral vertebra i s similar in size to the last lumbar, the other four become progressively smaller. The vertebral canal i s small and the spines are higher than those in the lumbar region. The cranial articular surfaces are lik e those of the lumbars, cranial and caudal articular pro-cesses are ve s t i g i a l . The ilium i s joined to the f i r s t sacral vertebra. No variations other than ones of size were found. Caudal Vertebrae-Seventeen to nineteen caudal vertebrae may occur. They become progressively smaller posteriorly. The vertebral oahal is incomplete but there are wide vestigial spines present. Cranial articulating processes are absent or minute and caudal articulating processes are absent. Chev-ron bones are present between the vertebrae but they do not form arches. There are many more caudal vertebrae present in townsendi and longicauda than in serpens but as some of the vertebrae were undoubtedly lost in the cleaning of the bones counts are useless. Serpens has short caudal vertebrae which become progressively shorter' and less well-developed posteriorly. Townsendi has more robust vertebrae which are s t i l l relatively short and show the same tendency to decrease in size. The meta-pophyses, spinous processes and zygopophyses are better-developed in townsendi than in serpens but the development of these features is decided-ly greatest in longicauda; the spine being much more obvious and enlarged. There i s less tendency for the vertebrae to become shorter posteriorly in the latter species, instead they become more delicate while remaining approximately the same length. Sternum-The sternum of serpens consists of a manubrium, four similar sternebrae, and a xiphisternum. There is no evidence of intercalary bones. A slight heel i s present and serves to increase the area of attachment of the pectoral muscles which are particularly heavy in this species. In longicauda and townsendi the manubrium and the sternebrae have fla t dorsal surfaces, i.e., the heel is missing suggesting a correlation with the development of the pectoral muscles. Manubrium-The manubrium sterni i s almost heart-shaped; with a well-developed jugular notch, a wide anterior surface, a caudally-directed apex, and smooth, sloping sides. Clavicular facets are present on the anterior sur-face at the angle of the jugular notch and there is a roughened pi t for the f i r s t costal cartilage on the lateral margin just below the enlarged anterior portion. The second costal cartilage i s attached at the junction of the manubrium and the f i r s t sternebra. The manubrium i s elongate in townsendi and longicauda but i t does not possess the heart-shaped anterior end found in serpens. The facets which show the position of the clavicular and costal cartilages are located in approximately the same position in a l l of the species. 1 3 . Sternebrae-In serpens the four sternebrae are alike in shape but they become progressively smaller posteriorly. They are a l l definitely heeled. In townsendi and longicauda there i s the same tendency to de-crease in size but there i s no sign of a heel. Xiphiaternum-The xiphisternum i s elongate, slender, and continuous caudally with a large heart-shaped cartilage. It forms a very slight angle with the last steraebra^and i s directed caudally. * n l ° n g i c a u d a and townsendi the xiphisternura is more attenuate and delicate. Ribs-There are thirteen ribs in serpens a l l of them double-headed but the last. The capitulum articulates with the centrum and the tuberculum with the transverse process. The heads tend to become united on the more posterior ribs. The f i r s t rib i s much shorter and somewhat stouter than the others. A l l of the ribs bear costal cartilages, those of the f i r s t seven reaching the sternum and that of the eighth being united to the seventh. No interspecific variations in the ribs were noted. Skulls-The skulls of serpens, townsendi and longicauda are very similar, any differences which do occur being very slight. The skull of serpens is broader in proportion to its total length, the width being 49$ of the length as compared to 44# in townsendi and longicauda, and the greater width occurs in the parietal bones rather than in greater curvature of the zygomatic arch. 14. Dorsal View-The nasal bone i s relatively shorter in serpens, being 27% of total length of the skull as compared to 31% in townsendi and 29% in Iongicauda. The surfacesof the nasal bones are ridged slightly in serpens but smooth in the other species. The frontal bones become very narrow in the central part of their length in each case, this i s less evident in longicauda than i t i s in the other species. The parietals are almost square and are large in a l l species. The interparietal i s rectangular with the vertical azis shorter. The lines of suture of the zygomatic arch are indistinct. The lambdoidal crest i s more distinct in townsendi and longicauda than i t i s in serpens. As a positive indication of muscle attachment there are slight processes just ventral to the crarilo-vantral border of the infraorbital foramen from which the masseter superficialis originates, this process i s decidedly better-developed in serpens than i t i s in the other species. This fact i s in accordance with the greater development of fibres in this portion of the muscle in that species. The zygomatic arch i s relatively the same width in a l l of the species which means that there is no bone formation to correspond to the greater development of the other parts of the masseter in serpens. In profile the three skulls are very similar but the anterior portion of the skull i s more curved in serpens. There are no v i s i b l e var-iations in the infraorbital ridging which agrees with Howell's statement (1926) that the "infraorbital ridging i s not the direct result of muscular stimulus". No evidence of the amount of development of the temporalis muscle is v i s i b l e on the skulls. However, the posterior divergence of the zygomatic arch appears to be slightly greater in townsendi than in serpens which may result from the exclusion of the temporalis muscle over a larger 15. area in the former species. Ventral Aspect-The tympanic bullae may be described as ovoid in a l l of the species but their length compared to total length of the skull varies, measuring 29% in serpens, 32% in townsendi and 31% in longicauda. The incisive foramina are a l l decidedly elongate and similar to that illustrated by Howell (1926) in which he attributes the length to the size of the Jacob-sen's organs of the nasal passages since the nasal arteries and the naso-palatine arteriesjwhich also pass through the foramen, are small. The pterygoid plates are similar in size and shape in a l l of the species but the mesopterygoid appears to be slightly longer in serpens. The occipital surface in each case exhibits an anterior inclination which is considered to be characteristic of rodents with fossorial habits but as i t is equally well-developed in the forms which are not regarded as being subterranean this structure possibly is not related to habitat. The occipital is known to vary in other forms with regard to angularity and definition of the processes and i t seems quite likely that the slight differences which do occur should be attributed to age. The mastoid process is heavier in serpens which, according to Howell, (1926) is an indication of better development of the sternomastoid, cleidomastoid, and complexus muscles in this species. Mandibles-The mandibles are very similar in the three species. The coro-noid process varies slightly being a tr i f le more hooked in serpens and longer in townsendi. The angular process is least thickened at the pos-terior border in serpens but is the same general shape in each specimen. Both the dorsal and the ventral masseteric ridges are always heavy. The symphysis menti i s movable and the two halves of the lower jaw are evident-ly connected by an elastic material most of which dissolves when the bones are boiled. The point of attachment of the M. transversus mandibularis i s marked on either side by a slight fossa. Teeth-The incisors are broader and shorter in M. serpens. The length of tooth row of, the cheek teeth in percent of total skull length measures 21.6% in serpens, 24.8% in townsendi and 22% in longicauda. The maxillary tooth row i s nearly parallel in each case. There are no noticeable differences in breadth of the molars in any of the species. The skull of richardsoni i s much larger with heavier ridging, this is especially true of the lambdoidal ridge and the occipital condyles. The nasal bones are a different shape in this species being much broader in the anterior end in proportion to the width at the posterior end. The zygomatic arch i s broader giving the entire skull the appearance of breadth. The parietals are longer and narrower and the interparietal bone i s much shorter in the horizontal axis. The tendency for the frontals to become narrow in the central region i s most evident in richardsoni. This ventral surface i s more similar to the other skulls but the diastema i s longer in percent of total length being 40.5% as compared te 29% in serpens and 34 and 31% in townsendi and longicauda respectively. The incisive foramen i s small measuring only 2 mm.^. in length as compared to 5 mmij. in townsendi and 3.5 mm?,!, in serpens. The Goronvold process i s shorter in richardsoni. Both the upper and lower incisors are longer, and the former project noticeably beyond the plane of the extremity of the rostrum. 17. Pectoral Girdle and Limb , Scapula. The scapula i s shaped roughly like a 30-60-90° triangle with the vertebral border convex and forming an angle greater than 90° withHhe anterior border. The cranial border is straight and forms a sharp angle with the anterior border. The.scapula tends to be elongated rather than broadened. The large and very distinct coracoid process is fused with the scapula but does not show any tendency to divide into two centres, a large supraglenoid tuberosity projects laterally at its base. The glenoid fossa i s ovoid (much longer than wide), the coracoid forming slightly less than one-third of the fossa. The infraglenoid tuberosity is less well-developed. The insertion of M. teres major i s marked by a minute third fossa on the lateral surface, which is separated from the infraspinous fossa by the axillary ridge. The vertebral border of the scapula is thickened probably in re-sponse to the insertion of the stout serratus anterior, levator scapulae and rhomboideus muscles. The costal surface is almost flat, but with a shallow groove opposite the spine. There is no evidence of secondary ridging lateral to i t . The lateral surface is divided by the spine into supraspinous and infraspinous fossae. The supraspinous fossa is the larger and i s roughly triangular in shape, with the portion near the coracoid much reduced. The spine is very prominent being approximately one-half the width of the scapula in height; possibly this development is in response to the sub-scapular group of muscles which are enlarged in serpens. The acromion process is represented by a thickening on the end of the spine equal to 18. approximately one-third of the length of the latter. A fairly prominent metacromion process is present. Length of the scapula varies with age (Howell 1924) but in the specimens measured the following percentages of functional arm length were obtained: 50.8% in serpens; 48% in townsendi; 49.9% in longicauda; and 42% in richardsoni. On this basis serpens has decidedly the longest scapula, a modification that may be correlated with the large muscle attachments found on i t . Howell (1924), mentions that young animals have a shorter scapula and this probably explains the divergence found in my specimen of richardsoni. The scapulae tend to be the same general shape in a l l of the species but in townsendi and longicauda the width at the vertebral.border i s considerably greater in proportion to length that i t i s . i n serpens. The vertebral border i s more thickened in serpens than in the other species. The coracoid is always fused with the scapula. -Clavicula The slender clavicle measures 6 - 6.25 mm,, in serpens. The sternal end is club-shaped and articulates with the anterior (cranial) surface of the manubrium sterni at its centre. Only clavicles of serpens and iownsendl were available for com* parison and no significant differences appeared. Functional length of the fore-limb-The scapula can be thrust forward and backward through muscle action,ijs., i t i s movable and should be considered as forming part of the functional length of the fore-limb. This movement compensates for the greater length of the hind limb which Is attached to an immovable pelvis. However, functional limb length is measured by Howell (1926) as the length 19. from the scapulces-humeral joint to the sole of the foot since this is a plantigrade animal. He feels that a definite point on the scapula could i not be designated from which to take measurements and hence it is better to measure from the joint. Actually the length taken i s only the sum of the length of the humerus and radius. Thi3 length measured 22 mm-}, in serpens; 25.3 mm,;, in townsendi and 25.8 mm;;., in longicauda. Humerus- . In serpens the humerus is long and slender with well-developed muscular processes. The head is large and ovoid forming an angle of about 150° with the shaft. The neck is poorly-developed. The greater tubercle is large and has a groove in the centre into which M. supraspinatus and M. infraspinatus insert, a depression markaing the point of insertion of U. teres major i s also present. The intertubercular sulcus, in which the tendon of the long head of M. biceps l i e s , is fairly deep. The lesser tubercle is about one-half the size of the greater tubercle. A poorly-defined depression marks the area of insertion of the tendon of M. sub scapular is". The shaft is slender. The lateral ridge extends more than one-third (less than one-half) of the distance from the head to the condyles and has faintly-marked areas of muscle attachment. The median ridge is indistinct, poorly defined scars marking the points of insertion of the tendons of M. teres major and M: latissimus dorsi. The lateral epicondylar ridge is distinct and the lateral epicondyl wellrdeveloped although small. The medial epicondyl is larger and a distinct roughness marks the point of attachment of the coracobrachial muscle. The condyles are separable into a large medial trochlea which articulates with the semilunar notch 20. of the ulna and a capitulum of similar size that articulates with the head of the radius. In percentage of functional limb length the humerus measures 54% in serpens; 47% in townsendi; 44% in longicauda; and 50% in richardsoni. In serpens the humerus i s longer and more slender than i t is in townsendi and longicauda although i t i s s t i l l a slender bone in the latter species. In richardsoni i t i s more massive and i s an entirely different shape. The head of the humerus i s approximately the same relative size in serpens, longicauda, and townsendi but i s much larger in richardsoni. This relationship also applies to the development of the greater and lesser trochanters. The neck shows a greater degree of development in townsendi and longicauda than i t does in serpens. The medial ridge appears to be better-developed in townsendi and longicauda than in serpens but this may have been only a relative difference. The lateral ridge i s large and well-developed in a l l species, that of longicauda being the longest. The con-dyles are always separable into a trochlea and capitulum and the medial epicondyl is always large and distinct. In percentage of functional arm length the widths of the condylar borders measured 25% in serpens; 30% in townsendi; 28% in longicauda; and 32.6% in richardsoni. Ulna-The ulna i s long and slender with a long olecranon process that measures one-sixth of i t s total length and provides a large area for inser. tion of M . triceps. The caudal end of the process i s curved mediad. The semi-lunar notch i s deep, semi-circular, and col l a r - l i k e , with a radial facet on the craniolateral border which receives the head of the radius. i t / The coronoid process located on the craniodistal margin of the notch pro-jects medially. The ulna i s excavated on the late r a l surface forming a 21. small, short lateral fossa which is very incomplete. The distal end of - the ulna has a groove through which passes ten-dons of most of the extensor muscles and another which permits the passage of the tendon of M. extensor ulnaris. The styloid process is V-shaped and f i t s into a depression between the triquetum and pisiform bones. In percent of functional limb len$h the ulna measured 63.6$ in serpens; 64.8% in townsendi; 65% in Iongicauda; and 58% in richardsoni. The percent of the ulnar length which is represented by the olecranon process measured 17.8% in serpens; 13% in townsendi; 12% in longicauda; and 14.4% in richardsoni. The variations between species are very slight, but the same rela-tionships that were found in the humerus re-occur here. . The only differ-ence worthy of note i s in the t r i o c c i p i t a l process which i s deeper and therefore better-developed in serpens. Radius-The head of the radius i s ovoid with a groove on the lateral sur-face which articulates with a small ridge on the ulna and thus reduces movability of the radio-ulnar joint. The neck of the radius is probably attached to the ulna by ligaments but these were not seen. However, practically no independent movement can be possible since the bones re-mained together even when cleaned by boiling. The cranial border i s oblique and extends caudally from the proximal end for about one-half of the length of the radius. The tendon of M. adductor p o l l i c i s inserts into a slight groove on the lateral sur* face about 1 mm. distal to the oblique l i n e . The body of the radius i s very slender but the tuberosity i s well marked and i s situated on the medial border near the head. The 22. "pseudostyloid" process described by H i l l (1937) is poorly differentiated. The volar surface i s nearly f l a t (not concave). The articular grooves are similar in each of the species* In per cent of ulnar length the radius measured 78% in serpens; 79$ in townsendi; 79.9$ in longicauda; and 78$ i n richardsoni. Carpus -The following diagram illustrates the relationships of the nine bones of the carpus: Radi us Ulna pPisiform Scapholunaris Triquetrum (navicular & lunate) I Centrals Capitatum Lesser'' Greater — ^ Palciformis Multangulum Mufttangulum MC I MC I I MC I I I MB IV MC V In each specimen examined the bones were similarly arranged. The bones are larger i n longicauda and townsendi which have much larger feet. Metacarpus -The f i r s t metacarpal is about the same length as the f i f t h and about two-thirds of the length of the third and fourth. The second i s slightly longer than the f i r s t . Proximally the metacarpals articulate with the distal carpal bones. Small sesamoid bones occur at the carpophalangial joints. Phalanges -The f i r s t two phalanges of each digit are elongate, the second being approximately two-thirds of the length of the f i r s t . The fourth digit i s the longest and bears the largest claw. The latter relationship i s not as pronounced in longicauda or townsendi as i t i s in serpens. 23. Pelvic Girdle and Limb Os Coxae-The os coxae are long and slender with well developed muscular processes. The sacrum and ilium are fused, the l a t t e r being attached to the f i r s t sacral vertebra. The ischium measures approximately one-half the length of the ilium. The coxal bones are united by a short symphysis which is approximately one-fifth the length of the pubis. The large and deep acetabulum i s situated caudal to the middle of the innominate bone and i s formed by the junction of the ilium, ischium and pubis. The lips of the cup are incomplete on the caudoventral side at the point of attachment of the ligamentum teres. The cavity at the "bottom" of the acetabulum is i r -regular in shape. The triangular obturator foramen i s situated ventral and caudal to the acetabulum with i t s main axis horizontal. The ilium extends from the cranial end of the acetabulum in a cranial direction. The cranial end i s thickened and flattened. The crest i s produced lat e r a l l y forming a recurved spine from which originates part of M . gluteus medius. The i l i a c ridge i s poorly defined. The gluteal fossa is not very deep but i t i s f a i r l y wide with a definite ridge separate ing the areas of attachment of M. gluteus medius and M . gluteus minimus. The dorsal and ventral borders of the ilium are nearly parallel. The dor-sal border is indented near the caudal end by the sciatic notch. On the ventral border opposite the sciatic notch there i s a slight indentation where M . psoas major inserts; and a slight iliopectineal eminence caudal to the latter. The ischium forms the remaining part of the dorsal and caudal parts of the hip bone. The ischial tuberosity does not seem to be divided into dorsal and ventral tubercles but the dorsocaudal margin i s thickened 24. to form a distinct ridge. The dorsal tuberosity i s large and prominent. The ischial spine i s poorly developed. The pubis extends in a caudal direction with a slender body which is apparently completely separated from the acetabulum. The cranial limit is marked by an iliopectineal eminence and the ventral border is oblique o forming an angle of about 15 with the body. This angle straightens out in the area of the symphysis. In percent of functional limb length the os coxae measure 48.2% in serpens; 40.6% in townsendi; 48.8% in longicauda; and 47.6% in richarcU soni. In percent of the length of the innominate bone the ilium measures 54.8% in serpens; 60.0% in townsendi; 56.1% in longicauda; and 54.8% in richardsoni. Any specific variations which .are found in the pelvic girdle are not as marked as those found in the pectoral girdle and limb. The pubic symphysis is closed in a l l of the species. The caudal border of the ischium i s not straight in longicauda but rather forms an angle of about 135° at the central point. This variation i s not as marked in townsendi. Functional length of the hind limb-Functional length of the hind limb was computed according to Howell's plan (1926). This i s a purely arbitrary system whereby the lengths of the femur and tibio-fibula are summed and added to one-half of the length from the heel to the distal end of the longest metatarsal. Measured in this way the limb length measure 33.7 mm,:, in serpens; 41.8 mm :. in townsendi; and 42 mm-,], in longicauda. Femur-The femur which i s long and relatively slender has large process-es for muscle attachments The neck forms an obtuse angle with the shaft 25. and i s g r e a t l y c o n s t r i c t e d . Ligamentum t e r e s i s attached to a deep and c i r c u l a r p i t on the head. The greater t r o c h a n t e r i s l a r g e ; the l e s s e r o t r o c h a n t e r smaller and with i t s main axis at an angle of about 60 to the surface of the femur. The t r o c h a n t e r i c fossa i s l a r g e , deep and i r r e g u l a r . The t r o c h a n t e r i c c r e s t i s complete, i . e . , i t connects the t r o c h a n t e r s e There i s no i n t e r t r o c h a n t e r i c l i n e on the c r a n i a l surface of the c r e s t . A t h i r d t r o c h a n t i c ( o r l a t e r a l r i d g e , H i l l (1937)) i s present and extends about t h r e e - e i g h t h s of the t o t a l l e n g t h from the proximal border caudad. There are s e v e r a l sunken l i n e s on the medial surface d i s t a l to the l e s s e r t r o c h a n t e r which mark the areas of i n s e r t i o n of tendons of M. i l i a c u s , M. p e c t i n e u s , and U . adductor Iongus. The proximal end i s bent s l i g h t l y c r a n i a d but the remainder of the shaft i s n e a r l y s t r a i g h t . The d i s t a l end i s enlarged to form two condyles which are about equal i n s i z e . The p o p l i t e a l groove i s deep and U-shaped and the p a t e l l a r groove shallow and r a t h e r narrow. N e i t h e r the l a t e r a l n o r the medial epicondyl has become markedly roughened by the attachment of l i g a m e n t s . The femurs of the three species show no extensive d i f f e r e n c e s i n s t r u c t u r e ; the head i s s l i g h t l y more overhanging i n serpens than i n townsendi or l o n g i c a u d a . In percent of f u n c t i o n a l l imb length the femur measures 37% i n serpens; 40.1% i n townsendi; 39.7% i n l o n g i c a u d a ; and 36% i n r i c h a r d s o n i . T i b i o - f i b u l a -The t i b i a and f i b u l a are ankylosed f o r n e a r l y o n e - h a l f of t h e i r lengths the proximal 55% of the l e n g t h of each bone being f r e e . The a r t i c u l a r surfaces f o r the femur are l a r g e and the medial and l a t e r a l con-dyles are of approximately the same s i z e . The a r t i c u l a r surface i s concave i n t r a n s v e r s e s e c t i o n . The t u b e r o s i t y i s p a r t of the common proximal 2 6 . e p i p h y s i s . The p o s i t i o n o f t h e i n s e r t i o n o f t h e l a r g e p a t e l l a r t e n d o n i s i n d i c a t e d by a s c a r be tween t h e c o n d y l e s . The body o f t h e t i b i a be tween t h e p r o x i m a l e p i p h y s i s and t h e u n i o n w i t h t h e f i b u l a i s l o n g and e r e s c e n t - s h a p e d . The c r e s t ( o r l a t e r a l b o r d e r ) b e g i n s c a u d a l t o t h e t u b e r o s i t y and r u n s a l m o s t p a r a l l e l t o t h e l a t e r a l s u r f a c e . The a r e a o f i n s e r t i o n o f U . semimembranosus i s l o c a t e d n e a r t h e m e d i a l b o r d e r d i s t a l t o t h e e p i p h y s e a l l i n e and d i s t a l t o t h e l a t t e r a r e t h e i n s e r t i o n s o f M . g r a c i l i s a n d M . s e m i t e n d i n o s u s . The m e d i a l c r e s t b e g i n s d i s t a l and c a u d a l t o t h e c o n d y l e s and c u r v e s f i r s t m e d i a d and t h e n l a t e r a d , becoming i n d i s t i n c t . R e l a t i v e t o t h e t i b i a , t h e f i b u l a i s more c a u d a l t h a n l a t e r a l i n p o s i t i o n and t h e h e a d f o r m s p a r t of t h e p r o x i m a l e p i p h y s i s . N e a r t h e e p i -p h y s e a l l i n e t h e r e i s a r o u g h e n e d a r e a w h i c h m a r k s t h e p o i n t of a t t a c h m e n t o f t h e f i b u l a r c o l l a t e r a l l i g a m e n t . The body o f t h e f i b u l a i s v e r y s l e n d e r and t h e i n t e r o s s e o u s c r e s t i s p o o r l y d e f i n e d . T h e c o j o i n e d t i b i a and f i b u l a i s l o n g and s l e n d e r . T h e r e a r e r i d g e s on t h e l a t e r o c r a n i a l s u r f a c e n e a r t h e m a l l e o l i w h i c h r e p r e s e n t t h e m a r g i n s o f t h e two b o n e s . The d i s t a l end o f t h e t i b i o f i b u l a c o n s i s t s o f b o t h g r o o v e s a n d m a l l e o l i . T h e t e n d o n of M . p e r o n e u s l o n g u s p a s s e s t h r o u g h an o b l i q u e g r o o v e on t h e l a t e r a l s u r f a c e . The m e d i a l m a l l e o l u s b e a r s two f a i n t g r o o v e s t h r o u g h w h i c h p a s s t e n d o n s o f M . f l e x o r t i b i a l i s and M . t i b i a l i s p o s t e r i o r . The d i s t a l end of t h e t i b i o f i b u l a a r t i c u l a t e s w i t h t h e t a l u s and c a l c a n i u m . The t i b i o f i b u l a i s l o n g e s t in s e r p e n s . M e a s u r e d in p e r c e n t o f f u n c t i o n a l l i m b l e n g t h t h e r e s u l t s a r e 52 .7% i n s e r p e n s ; 50.2% i n t o w n s e n d i ; 50% i n l o n g i c a u d a ; and 52% i n r i c h a r d s o n i . I t i s d e c i d e d l y l o n g e r t h a n t h e femur i n a l l c a s e s . The p e r c e n t o f t h e bone w h i c h i s s e p a r a t e i s 42 .8% i n a l l s p e c i e s . 27. Foot-The r i g i d fusion of the t i b i a and fibula probably has an appre-ciable effect on lessening the amount of rotation of which the foot is capable. Flexion of the toes makes measurement of the foot very d i f f i c u l t , however, the following measurements which have been converted to percent of functional limb length were obtained; 41.1% in serpens; 44.2% in townsendi; and 45% in longicauda. Tar sus-Tbe following diagram illustrates the positions of the eight bones of the tarsus: Tibio-fibula f Talus Calcaneum Cuboideum I Cuneiform III / \ Met. V Met. 17 Navicular \ I Cuneiform II I Met. I l l Tarsale medialis , . (T i b i a l i s ) Cuneiform I i \ Met. II Met. I Metatarsus-The f i f t h metatarsal i s the smallest being shorter than the f i r s t which articulates with the cuneiform and with the second metatarsal as described by H i l l (1937). phalanges-The phalanges of the hind foot are short bones. They are much shorter and more slender than the corresponding bones of the manus. 28. Branchiomeric Uusculature Masticatory group (fig. 15) supplied by the masticatory nerve (branch of trigeminal). M. masseter; a) M. masseter superficialiso is a sheet of muscle arising ventral to the infraorbital foramen and inserting on the ventral margin of the mandible and on the masseteric ridge. b) u. masseter lateralis profundus, pars anterior* arises from the rostrum and adjacent part of the zygomatic arch and passes to the lateral surface of the mandible (over a large area). c) M. masseter lateralis profundus, pars posterior- arises from the lateral and medial surfaces of the zygomatic arch and inserts on the angular process of the mandible. d) M. masseter medialis, pars anterior- arises from the medial surface of the zygomatic arch and inserts onto the coronoid process of the mandible. e) 11. masseter medialis, pars, posterior- arises from the angle of the zygoma (medial surface) and inserts on the ramus of the mandible (over a large area). The entire muscle is enclosed in a heavier fascia in townsendi and longicauda than i t is in serpens but in the latter species there are more muscle fibres present in the superficial part. In richardsoni the fascia is very much heavier as is also the platysma and the,/>^n,cu/</3 coLfnosus. The origin and insertion of each part is the same-in a l l of the species; a rule which holds for the majority of the muscles. M. temporalis- is a large and easily discerned muscle, especially in the region of insertion. It arises by an aponeurosis from the temporal ridge . 2 9 . lateral to the Interparietal with no sign of division into deep and super-f ic ial fibres. However two distinct areas of insertion occur, the super-ficial fibres insert on the tip of the coronoid process and the deep fibres on the medial surface of the same process. M. temporalis occupies a relatively larger area in townsendi and longicauda but i t contains fewer fibres. In richardsoni i t is decidedly smaller. M. pterygoideus externus- takes origin from the parapterygoid plate and fibres run lateralt and caudad to insert on the medial surface of the mandibular condyle. • It. pterygoideus in t emu a- is supplied by the medial branch of the mandibu-lar nerve. It arises from the pterygoid fossa, i . e . , the area between the lateral and medial pterygoid plates and inserts onto the medial surface of the angular process of the mandible. This muscle is decidedly heavier in serpens than in any of the other species. In townsendi i t arises from the lateral surface of the internal pterygoid plate. Considering the degree of development found in the ether masticatory muscles i t is a well-developed muscle in richardsoni. Mylohyoid group (fig. 16) supplied by the mylohyoid branch of the mandibular nerve M. mylohyoideus- arises from the mylohyoid ridge ( i . e . , the ridge ventral to the cheek teeth) of the mandible and inserts on the byoid arch. M. transversus mandibularis- is a poorly-defined muscle consisting of a few fibres which run transversely between the halves of the mandible. It is visible when the digastricus is cut away. In townsendi and longicauda i t gives the appearance of being 3 0 . slightly heavier although there are s t i l l very few fibres present. The origin and insertion is more clearly defined in the latter species. M. digastricus- i s divisible into two parts. The anterior belly arises from the body of the hyoid and from the arch ventral to this and i n -serts on the oblique ridge of the mandible and on the fossa caudal to the symphysis of the mandible. The right and left parts are separated. The posterior belly arises from the paraoccipital process (jugular process) and inserts en the hyoid arch by a tendon. Both parts are well-developed and they are separated by a constriction in a l l of the species although most noticeably largest in serpens. In richardsoni the muscle is more strap-like. Constrictor c o l l i group- supplied by the superficial branches of the facial nerve. M. platysma (fig. 15) shows no tendency to divide into sphincter c o l l i primitivus as described by Howell (1926). Two nil-developed heads to the sheet of muscle are found, one in the head and the other in the neck region. The cranial part arises from the mid-dorsal line (under levator auris longus), the base of the ear, and the parietal region (under funtalis), and I i n s e r t s ! o n t o the skin in the l i p region. The superficial layer lies in the ventral neck region where i t arises along the mid-ventral line from the pectoral region to the symphysis mandibuli, and inserts; on;the fascia of the masseter and of the shoulder. The muscle sheet adheres more closely to the skin in townsendi and longicauda than i t does in serpens making i t more difficult to define i ts limits. There is a much greater development of muscle fibres in the platysma of richardsoni than in any other species. 31. U-. postauricularis- (levator auris lohgus) is divided into cranial and caudal portions. The cranial part arises from the spines of the first three or four cervical vertebrae and inserts onto the base of the ear (anterior to the insertion of the caudal portion). The caudal part arises from the fourth and fifth vertebrae and inserts onto the base of the ear and onto the interparietal bone lateral to the ear. In townsendi this muscle is not as well-developed, i t inserts onto the skin of the ear and the area immediately cranio-dorsal to the ear. It is well-developed in richardsoni and is directed more laterally than in the other forms. H. auriculolabialia- not found. M . sternoauricularis- not found. U. nasolabialis- (zygomasticus) is a very indistinct muscle in serpens, consisting of a sheet of muscle which arises on the surface of the zygomatic arch and inserts at the upper corner of the mouth, i . e . , becomes attached to the mystacial pad of connective tissue. It is similar in townsendi and longicauda but i s less developed in richardsoni. M . maxillolabialis- (levator labix.) is well-developed and easily dis-tinguishable. It originates on the premaxilla in the region of M . orbicu-laris oculi and inserts on the mystidal pad of the upper l i p . It 1B a slightly heavier muscle in serpens than i t is in townsendi. I I . niaxilionasalis- (dilator noud',) passes under M . maxillolabialis after i t has arisen from the surface of the nasal bone directly in front of the eye and close to M . orbicularis oculi. It is a thin muscle which becomes tendinous and inserts on the lateral side of the anterior nasal opening. This muscle also shows the best development in serpens although the 32. variations are slight. Ii. orbicularis oculi- (fig. 15) is well-developed in serpens. Eibres run around the eye and adhere to the skin in some cases. The fibres arise from the hamulus of the lachrymal bone and from the fascia around the eye. In townsendi and longicauda the muscle is very poorly developed with the fibres less numerous and more difficult to follow. M. orbicularis oris- is well-developed in serpens and divided- into at least five slips which arise from the mandible, near the incisive alveoli, and from the medial line ventral to the nostrils. In townsendi and longicauda.the slips are not as readily separable. Ii. buccinatorius- runs under levator l a b i i (mazillolabialis) and in a direction at right angles to i t . It originates from the rostrum along a ridge formed by the upper incisor and from the area anterior te the later-alis profundus pars anterior portion of U. masseter. It inserts onto M. orbicularis oris. In townsendi and longicauda this muscle inserts upon M. orbicu-laris oris by five distinct slips that originate from a single head upon the rostrum. A thin muscle was found in serpens passing from under the edge of id. buccinatorius and fusing with U. orbicularis oris or passing under i t . This may correspond to the sphinter muscle of the pouch which is described by H i l l (1937). The muscle measured approximately 3.1 mm;;, in width and 6 mm;, in length. It was found in each specimen of serpens dissected. Hyoid constrictor group- supplied by the facial nerve and derived from the 33. ventral constrictor of tha hyoid arch. M. digastricus- is described with the mylohyoid group. M. stylohyoideus? was discovered only after some searching as there is very l i t t l e muscle fibre present, the larger portion of the mass consist-ing of connective tissue. It arises from the same area as the posterior belly of M. digastricus and passes under the latter muscle to insert on the hyoid. In townsendi and longicauda the muscle is poorly defined and in these species i t inserts on the greater cornea- of the hyoid in a position which is much more lateral than the area of insertion in serpens. M. jugulohyoideus- was not found. According to H i l l (1937) this muscle disappears with the loss of the stylohyal cartilage. M. stapedius- not found. Trapezius group- (fig. 18) These may be branchiomeric muscles with a . . . . . s secondary cervical innervation. Ontogenetically the group arises from the mesenchyme of the branchml arches. (Edgeworth, 1911; Lewis, 1910) M. sternomastoideua- is well-developed in serpens but shows no evidence of a double insertion as described by Greene (1935) in Rattus. It arises from the lateral surface of the manubrium sterol and inserts on the mastoid surface of the squamosal. In townsendi i t is a fairly well-developed muscle which arises from the clavicle at its point of junction with the manubrium sterni. M. cleidomastoideus- runs under M. sternomastoideus but is a thinner muscle. It arises from the lateral half of the clavicle and inserts with 34. M. sternomastoideus onto the tip of the mastoid process. U. cleido-occipitalis- (clavatrapezius) is a well-defined muscle which is readily discovered running obliquely between the trapezius and sterno-mastoid muscles. It originates from the lateral third of the posterior border of the clavicle and inserts on the superior nuchal l ine. M. trapezius- (fig. 18) is divisible into U. acromiotrapezius and M. spino-trapezius. M. acromiotrapezius- i s a well-developed, heavy muscle which arises from the superior nuchal line and from the mid-dorsal line over the first four thoracic vertebrae and inserts onto the spine of the scapula and the acromion process. In townsendi and longicauda the origin is from the same area as i t is in serpens but in the latter the muscle development is sufficiently great to give the effect of a ridge on either side of the mid-dorsal l ine. By comparison, the muscle is flat in townsendi and longicauda. M. spinotrapezius- is divisible in serpens into a small posterior and a large anterior portion. The smaller part arises from the sixth and seventh thoracic vertebrae and inserts on the spine of the scapula. The larger part, arises from the adjoining fascia and the mid-dorsal line and inserts on the spine and crest of the scapula. In townsendi and longicauda the posterior portion is long and strap-like and the anterior portion i s limited, arising from the fascia above the fifth thoracic vertebra and inserting with M. acromiotrapezius onto the spine of the scapula. In richardsoni only the posterior portion is present. 3 5 . Myotomic Musculature Lingual muscles- supplied by the hypoglossal nerve. The muscles of this group were not dissected out. Muscle8 of the Body  A. Dorsal Division The epazial muscles are supplied by branches of the dorsal rami of the spinal nerves. • Superficial spino-occipital group-M. spienlus- arises from the nuchal ligament and inserts on the superior nuchal line for almost its entire length. In the other species the origin is also frOm the nuchal ligament but i t is a heavier,and more obvious muscle in longicauda than in either serpens or townsendi. The difference may have been due, in part, to the preservative used, Sacrospinalis group- (fig. 17) M. sacrospinalis- is found in a position lateral to M. longissimus dorsi and consists of a number of slips which pass ever three ribs to insert on the fourth or onto the transverse processes of the lumbar vertebrae. The slips arise from the ilium, sacral and lumbar vertebrae, and the ribs. In townsendi there are not as many fibres to each slip of the muscle as there are in serpens and the anterior slips pass over only two ribs to insert on the third craniad. In richardsoni as in serpens the slips pass over three ribs. M. i l iocostalis- appears to be continuous with M. sacrospinalis. It is divisible into three parts, the lumbar part inserts on the last five or siz ribs, the dorsal part on the middle ribs, and the cervical part on 36. the first rib and the last two or three cervical vertebrae. Each slip passes over three ribs to insert on the fourth craniad. M. longissimus- consists of bundles of muscle fibres which arise from the mammillary processes and pass over two vertebrae to insert on the third craniad. Insertions occur on the caudal ten ribs and on the accessory processes of the thoracic and lumbar vertebrae. The several sub-divisions usually described for this muscle are not well-differentiated in the Microtyne species studied. M. extensor caudae lateralis- arises from the transverse processes of the last two sacral and the first few caudal vertebrae and inserts on the articular processes of the caudal vertebrae. The t a i l of serpens is re-duced and the muscles that move i t show a corresponding reduction. In townsendi the better developed muscle originates on the last three sacral vertebrae. M. spinalis dorsi- i s united with the fibres of U. semispinalis in both serpens and townsendi. Semispinalis group- (fig. 17) includes muscles which have fibres arising from transverse processes and inserting on the spines or lamellae of vertebrae. M. semispinalis capitis- is a large single muscle in serpens which arises from the transverse processes of the second to seventh cervical vertebrae and from the first to ninth or tenth thoracic vertebrae and inserts onto the middle part of the superior nuchal l ine. In townsendi and longicauda the muscle is roughly divisible into regions which probably correspond to the ones given by Howell (1926). The 37. anterior part or biventer cervicia arises from the cervical vertebrae including the axis and the posterior part from the first six or seven thoracic vertebrae. Both portions insert on the superior nuchal line. M. semispinalis- consists of fibres which interconnect the spinous pro-cssses. They arise from the mammillary processes of the lumbar and last few thoracic vertebrae and from the transverse processes of the other thoracic vertebrae. Generally the fibres pass over two vertebrae to in-i sert on the spine of the vertebra threecraniad but considerable variation occurs and in some parts only one segment is passed over. M< extensor caudae medialis- is continuous with M. semispinalis. The fibres arise from the spines of the last sacral and first few caudal vertebrae and pass over two vertebrae for insertion. The entire muscle shows greater development in townsendi and longicauda than i t does in serpens. B. Ventral Division Includes a l l the remaining skeletal muscles of both the body and the appendages. Branches of the ventral rami of spinal nerves supply these muscles. Cervical prevertebral group- supplied by branches which arise from the ventral rami as they emerge from the inter-vertebral foramina. M. rectus capitis anterior- arises from the margin of the transverse process and ventral arch of the atlas and inserts on the basal part of the occipital (anterior portion) and to the medial half of the jugular pro-cess (posterior portion). This muscle is less clearly-defined in townsendi and longicauda than i t i s in serpens. 38. M. longus capitis- (longus atlantis) originates from the carotid tubercle of the sixth cervical vertebra sn'dl the ventral parts of the fourth and fifth cervical vertebrae and inserts on the tuberosity near the occipito-sphenoidal suture. In townsendi this muscle covers a larger area but i t does not contain as many fibres as are found in serpens. U. longus c o l l i - is divisible into caudal and cephalic portions* The cephalic part arises from the ventral surface of the sixth cervical vertebra and from the ventral part of the transverse processes of the third to sixth cervical vertebrae and inserts on the ventral part of the axis. The caudal part arises from the heads of the first two ribs and from the lateroventral surface of the first three thoracic vertebrae and the last cervical vertebrae and inserts on the ventral tubercle of the sixth cervical vertebra. Lumbar prevertebral group-M. quadratus lumborum- arises from the last lumbar and the first sacral vertebrae by tendons and inserts on the bodies of the last two or three thoracic vertebrae and on the lateral surfaces of the first three lumbar vertebrae (no costal insertions were found). This muscle shows the least development in serpens. In townsendi and longicauda i t is a de-cidedly heavier muscle and this is also true to some extent of richardsoni. M. psoas minor- may have been derived from M. quadratus lumborum according to Gregory and Camp (1918). It arises from the iliopectineal eminence and inserts on the bodies of the first four or five lumbar vertebrae. 3 9 . Caudal flexor group-M. flexor caudae lateralis- arises from the pelvic surface of the ilium (origin is restricted to the sacrum) and inserts on the chevron bones of the t a i l and on the ventral surfaces of the caudal vertebrae. U. flexor caudae medialis- is divisible into thres parts. The cranial part originates from the radius and the transverse processes of the second and third sacral vertebrae. The intermediate part arises from the bodies of the second to fifth sacral vertebrae and the caudal part, which is small, arises from the first two caudal vertebrae. A l l of the parts in-sert with M. flexor caudae lateralis on the chevron bones of the t a i l . This muscle is developed most in longicauda and least in serpens. Medial ventral cervical group-The muscles of this group develop ontogenetically from a single rudiment (Edgeworth, 1916). The geniohyoid is supplied by the hypoglossal nerve and the infrahyoid muscles are innervated by a special nerve which arises from the loop between the first and second cervical nerves independent of the hypoglossal. U. geniohyoideus- i s well-developed in serpens but i t is not a large muscle. It arises from the hyoid arch and inserts on the symphysis of the mandible In townsendi and longicauda the origin i s from the posterior corn& of the hyoid., M. sternohyoideus- is the chief muscle lying ventral to the trachea. It arises from the posterior end of the manubrium sterni and from the costal cartilage of the second rib and inserts onto the lower border of the body of the hyoid. 40. As is the case with many of the cervical muscles M . sternohyoideus i s much more clearly defined in townsendi and longicauda than i t is in serpens. The muscle i s largest in richardsoni, M. sternothyreoideus- is a well-developed muscle which arises from the lateral part of the thyroid cartilage and inserts on the hyoid. M. omohyoideus- is well-dofined and fairly large in serpens. It arises from the cephalic border of the scapula and inserts onto the hyoid (deep and lateral to U. sternothyreoideus). Medial thoraco-abdominal group-M. rectus abdominus- is attached to the crest of the ilium, the first rib, and the manubrium sterni. The two halves of the muscle are divided in the abdominal region by the linea alba. The muscle on each side arises by several slips some of which cress the mid-ventral l ino. The external oblique, internal oblique and transversalis abdominalis muscles are a l l very thin and practically impossible to separate in any of the species. The external oblique is continuous with M. serratus anterior, i . e . , i ts fibres interdigitate with those of the latter muscle and i t inserts into the crest of the ilium and the body of the pubis. It spreads out like a thin fan over the ventral and lateral surface of the body. Lateral cervical group- supplied by dorsal branches of the ventral rami of the cervical nerves. M. scalenus is also inner-vated by the first two or three intercostal nerves. M. rectus capitis lateralis- arises from the transverse process of the atlas and inserts on the medial ridge of the jugular process. M. scalenus- is divisible into two part3. The dorsal portion arises by 41. a tendon from the transverse process of the atlas and Inserts on the third, fourth and fifth ribs. The vsntral portion arises by a tendon from tho dorsal muscle and from the transverse processes of the last six cervical vertebrae and inserts on the dorsal part of the last six cervical vertebrae and on the cranial surface of the first r i b . No appreciable variations were discovered in any of the muscles of the lateral cervical or the lateral thoracoabdominal groups. Lateral thoracoabdominal group- (fig. 16) supplied by tho intercostal and first lumbar nerves and by the inguinal nerve. M. serratus posterior superior- arises.at the mid-dorsal line between and beneath the scapulae and over the first thoracic vertebrae and inserts on the cranial side of the fourth to ninth ribs. M. serratus posterior inferior- arises from the last thoracic and the first two lumbar vertebrae and inserts onto the caudal surfaces of the last four ribs. M. sternocostalIs- not found. Mm. subcostalis- (intercostalis intern!) arises from the pleural surfaces of a l l but the first rib and from the transverse processes of the first two lumbar vertebrae and inserts on the second ribcraniad. Mm. levatores costarum- (intercostalis extern!) arise by tendons from the accessory processes of the thoracic vertebrae and run caudovontrad to insert on the ribs. M. transversalis thoracis- arises from the xiphisternum and the last three sternebrae and inserts' on the costal cartilages of the second to tenth ribs. 42. Appendicular Musculature The limb muscles are divided into extensor and flexor systems which are supplied by dorsal and ventral branches, respectively, of the brachial and lumbodorsal plexus. Most of the muscles attached to the pectoral girdle show a marked increase in size in serpens. Muscles of the Pectoral Girdle and Limb  A. Extensor System  Costo-spino-scapular group- (fig. 19) M. levator s c a p u l a B - i s quite distinct from M. serratus anterior but con-tinuous with i t in serpens. It arises from the transverse processes of the first four or five cervical vertebra* and inserts onto the vertebral border of the scapula. U. serratus anterior- arises by fleshy digitations from the first to sixth or seventh ribs and inserts onto the vertebral border of the scapula posterior to levator scapulae. M. levator scapulae and M. serratus anterior are two not distinctly separable muscles in townsendi and longicauda. They are not, however, a combined single muscle such as that described by H i l l (1937). M. rhomboideus- is a single, large muscle in serpens which arises from the spines of a l l the cervical and the first four thoracic vertebrae and inserts on the lateral margin of the vertebral border of the scapula. In townsendi and longicauda i t is a smaller and thinner muscle. In serpens the muscle is actually larger than i t is in townsendi although the latter species is one and one-third times as large as the former by volume. 43. M. occipitoscapularis- (rhomboideus capitis) arises from the lambdoidal ridge and inserts on the vertebral border of the scapula. This muscle is relatively heavy in a l l of the species. M. omocervicalls- (levator claviculae) arises from the transverse process # of the atlas and inserts on the metacromion process of the scapula, Latissimus-subscapular group- supplied by the subscapular nerves. M. latissimus dorsi- arises from the spines of the last two or three thoracic and the f irst two lumbar vertebrae and inserts through the axilla between the extensor and flexor muscles onto the shaft of the humerus. In townsendi this muscle inserts onto the axilla by three widely-separated heavy digitations rather than by a single insertion as found in serpens. M. teres major- originates from the axillary border of the scapula and inserts onto the medial ridge of the humerus. In serpens i t is slightly better-developed than in the other species. M. subscapulars- can be divided into four bipennate parts which arise from the subscapular fossa and insert by tendons onto the lesser tubercle of the humerus. Deltoid group- (fig. 18) supplied by the axillary nerve. M. deltoideus- is divisible into two parts in serpens; M. acromiodoltoideus and M. spinodeltoidaus. M. acromiodeltoideus- arises from the lateral half of the clavicle and the acromion process of the scapula and inserts onto the deltoid ridgo of the humerus; In serpens this muscle i s very well developed and covers the entire lateral portion of the shoulder. In townsendi and longicauda 4 4 . i t does not cover as great an area and consequently is not as prominent. M• spinodeltoideus- is a small rather indistinct muscle which arises from the spine of the scapula and inserts onto the deltoid ridge beneath M. acromiodeltoideus. It displays no significant differences in .the species examined, M. teres minor- arises from the ventral part of the axillary border of tho scapula and inserts onto the greater tubercle of the humerus. It was found to be. a narrow but distinct muscle in a l l of the species. Suprascapular group- supplied by the suprascapular nerve. M. infraspinatus- is a bipennate muscle which arises from the whole of tho infraspinous fossa and inserts onto the greater tubercle of the humerus. In townsendi and longicauda i t is not as distinctly bipennate as i t is in serpens. M. supraspinatus- is also bipennate but i t is a heavier muscle than M. infraspinatus. It arises from the whole of the supraspinous fossa and the spine of the scapula and inserts by a tendon onto the greater tubercle of the humeruso Triceps group- supplied by the radiaal nerve. M. triceps brachii- arises by three distinct heads in serpens. Caput Iongum is the largest part of the muscle but i t i s not separable into superficial and deep parts as described by H i l l (1937) in Thomomys. It takes origin from the axillary border of the scapula for most of its length and from the infraglenoid tuberosity and inserts with Caput laterals and caput mediale onto the olecranon process of the ulna. In serpens the 4 5 . insertion is extensive. Caput laterals arises from the greater tubercle of the humerus and caput mediale from the medial surface of the humerus caudal to the insertion of M. teres major. The same three heads are found in a l l of the species but varisu* tion in size of the belly of the muscle occurs, the greatest development being found in serpens. Triceps shows the same tendency to shortening in serpens that is found in a l l the muscles of the brachium. M. anconeus- is a rather small muscle, arising from the caudal surface of the lateral epicondyle and inserting onto the lateral surface of the olecranon process. In townsendi and longicauda this muscle appears te be relatively larger but there is l i t t l e actual difference in size. M. dorsoepitrochlearis- inserts with M. triceps onto the olecranon process. In townsendi and longicauda the two are indistinct, but in serpens the two muscles are completely separable. The dorsoepitrochlearis takes origin from the fascia of M. latissimus dorsi and M. teres major. In townsendi and longicauda no origin from the fascia of M. teres major was found. Extensor group of the fore-arm- extensor carpi radialis is supplied by the radial nerve before i t divides. The other two muscles are supplied by the deep ramus of the dorsal interosseous nerve. M. brachioradialis- not found. M. extensor carpi radialis, longus- is a slender but well-defined muscle which arises from the lateral epicondylar ridge of the humerus. It is 46. continuous with a tendon which passes over the carpus medial to i ts centre and inserts onto the radial surface of the second metacarpal. In townsendi the muscle is larger than that found in serpens and insertion takes place near the base of the second metacarpal. U. extensor carpi radialls brevis- arises from the lateral epicondylar ridge superficial to M. extensor carpi radialis longus with which i t passes over the carpus to insert on the dorsal surface of the third meta-carpal. This muscle is broader in townsendi than i t is in serpens. M. supinator- arises at the elbow joint by a stout tendon which is attached to the radial collateral ligament and inserts into the cranial border of the radius. This ia the deepest muscle of the antibrachium and i t is not particularly well-developed in any of the species. Iii. abductor pol l ic is- arises from the ulnar surface of the radius and from the body of the ulna and inserts by a double tendon which passes through a groove in the wrist to the first phalanx of the pallex and the first metacarpal. The other extensor muscles of the fore-arm are similar in a l l of the species but as the tendons are smallest in serpens they are most difficult to find in that species. In richardsoni the foira feet are large and the tendons are correspondingly developed. M. extensor digitorum communis- arises from the lateral epicondyle of the humerus and inserts onto digits two to five at the bases of the distal phalanges by tendons which pass under the transverse ligament. M. extensor indicis et poll icis- arises from the lateral epicondyle and radial collateral ligament and inserts onto the dorsum of the pollex. A 47. second slip joins part of U. extensor digitorum communis which passes to the second digit. The tendon of this muscle passes over the tendon of U. extensor carpi radialis brevis on the manus. M. extensor digit! quinti proprius- is a slender muscle which arises from the lateral epicondyl of the humerus and inserts into the second phalanx of the fifth digit, its tendon passing under the transverse ligament. A slip to the fourth digit may also exist but i t was very slender and indefinite. M. extensor carpi ulnaris- arises from the distal part of the lateral epi-condyl and from the lateral ridge of the ulna and inserts by a tendon, which passes along a groove on the ulnar side of the wrist, to an area near the base of the fifth metacarpal. 48. Fleior System Pectoral group- (fig. 18) supplied by the two anterior thoracic nerves with the exception of U. subclavlus which is supplied by a special nerve. These muscles are much larger in serpens than in any of the other species. When the muscles were dissected from each specimen the volume of muscle from serpens was found to be practically identical with that from the specimen of townsendi although the latter had a body vol-ume almost one and one-third times as great as that of the former. U. ectopectoralis (pectoralis major)- is divisible into superficial and dsep parts. This is a very heavy muscle with a superficial part arising from the manubrium sterni and inserting on the deltoid ridge of the humerus and a deep part arising from the manubrium and from the first four sternebrae and inserting on the deltoid ridge and the fascia of the arm. The divisions into superficial and deep parts are more definite in townsendi and,longicauda than they are in serpens. M. entopectoralis (pectoralis minor)- is a large muscle which arises from the second to fifth sternebrae and the xiphisternum and inserts on the deltoid ridge and the coracoid process. In townsendi and longicauda no fibres take origin from the xiphisternum. M. pectoralis abdominalis- (xiphihumeralis) arises from the xiphisternum and runs deep to U. ectopectoralis to insert on the lateral ridge of the humerus. 49. M. cutaneus maximus- is part of the panniculus carnosus which arises from the humerus and the surface of M. ectopectoralis by fibres which converge in the glutial region and at the base of the t a i l . This muscle was largest in richardsoni. Flexor group of the arm- supplied by the musculocutaneous nerve with the exception of some fibres of M. brachialis which are innervated by the radial nerve. M. coracobrachialis- arises from the coracoid process of the scapula to-gether with the short head of biceps brachii, and inserts on the distal two-fifths of the median ridge of the humerus caudal to the insertion of M. teres major. No distinct separation into two parts was found. In townsendi and longicauda the muscle covers a larger area than i t does in serpens. M. biceps brachii- is divisible into two parts. Caput Iongum takes origin in the glenoid fossa and inserts on the tuberosity of the ulna. Caput breve arises from the coracoid process of the scapula and inserts into the tuberosity of the radius. In townsendi the division is not as distinct as i t is in serpens. U. brachial!8- takes origin from the lateral surface of the humerus and the base of the greater tubercle and inserts with caput Iongum of U. biceps on the ulna. In townsendi and longicauda two origins were found which were similar to those described by H i l l (1937) for Thomomys. In these species a second origin lacking in serpens occurs from the cephalic border of the humerus. Flexor group of the fore-arm- supplied by the median and ulnar nerves. 50. M. epitroobieo-anconeus- is supplied by the ulnar nerve. It is a short muscle which arises from the medial epicondyl of the humerus and inserts on the olecranon process of the ulna. In serpens the tendency is for this muscle to be nearly covered by the insertion of the long head of M. tricep brachii. In townsendi the muscle is much more clearly-defined. Ii. flexor carpi ulnaris- is a single muscle in serpens which arises from the caudal surface of the olecranon and inserts oh the pisiform bone. The epicondylar bead described by H i l l (1937) was not found in any of the species and may represent the primitive condition. U. palmaris longus- arises from the medial epicondyl and inserts into the fascia of the hand. The insertion may extend to the falciform bone but i t was very indistinct in serpens. In townsendi i t was more readily followed and insertion on the falciform bone was observed. M. flexor carpi radialis- arises from the medial epicondyl and inserts on the second and third metacarpals but the exact source of innervation could not be determined. M. pronator teres- Is a stout, well-defined, muscle which arises from the medial epicondyl of the humerus and inserts on the cephalic border of the radius. M. flexor digitorum sublimis- corresponds to the condylo-ulnar flexor (described by McMurrich, 1902b). It arises from the medial epicondyl and inserts on the second phalanges of the middle three digits with the most ulnar part dividing to insert on both the fourth and fifth digits. This muscle really consists of three bipennate muscles bound together. Ii. flexor digitorum profundus- is divisible into three heads. The ulnar 51. head-arises from the proximal part of the medial surface of the ulna. The central (condylar) head arises from the medial epicondyl and the radial head from the proximal third of the radius. Tendons from the three heads unite at the wrist to form a common tendon which divides into five parts each of which inserts on the terminal phalanx of a digit. In townsendi and longicauda the insertion on the pollex was missing. M. pronator «uadratu3- runs in an almost transverse direction between the radius and ulna. It arises from the distal quarter of the ulna and inserts on the corresponding part of the radius. 52. Muscles of the Pelvic Girdle and Limb  A. Extensor System None of the muscles, attached to the pelvic girdle, exhibit variations in the three species which are sufficiently great to be con. sidered adaptive. N Iliacus group- (fig. 20) is analogous to the subscapular group of the fore-limb. Some fibres of M. pectineus are supplied by the obturator nerve and are probably derived from the adductor group; the other fibres of M. pectineus and the other members of the group are supplied by branches of the femor-al nerve. M. iliacus- is a stout, well-defined muscle which arises in the i l i a c fossa and from the transverse processes of the last lumbar vertebra. It inserts on the lesser trochanter, deep to M. psoas major and partly on the shaft of the femur, M. psoas major- may have been derived from M. suadratus lumbarum as de-scribed by Gregory and camp (1918). It arises from the iliopectineal iminence and inserts onto the borders of the first three lumbar vertebrae. M. pectineus- arises from the pubis craniad to the origin of U. adductor longus and inserts on the medial shaft of the femur. Gluteal group- analogous to the latissimus dorsi-subscapular muscles of the fore-limb. The muscles of this group occur in two layers. The lateral or superficial layer consists of M. fasciae latae, U. gluteus maximus, M. femorococcygeus and M. tenuissimus. U. fasciae latae is supplied by 53. terminal branches of the superficial gluteal nerve; M. tenuissimus by a branch of the common peroneal nerve and the others by the superior gluteal nerve. M• tensor fasciae latae- consists of a muscle sheet which, in serpens, is not as extensive as that described by Howell (1926) in Neotoma. or Greene (1935) in Rattus. It takes origin under M . gluteus maximus (and is continuous with it) at the crest of the ilium and inserts on the lateral fascia of the thigh and on the patella. Continuity with li. gluteus maxi-mus was also found in townsendi and longicauda. M. gluteus maximus- is a broad and relatively heavy muscle which arises from the spines of the third to fifth caudal vertebrae and inserts, by a tendon, on the greater trochanter of the femur. te. femorococcygeus- arises from the transverse processes of the f irst two or three caudal vertebrae (perhaps also the last sacral) and inserts on the distal part of the femur and perhaps on the patella. The limits of this muscle were very hard to discern. U. tenuiasimu3- not found. Medial layer: M. gluteus medius- l ies beneath M. gluteus maximus. It takes origin from the spines of the first four sacral vertebrae, from the fascia and from the crest of the ilium and inserts on the greater trochanter of the femur. U. piriformis- is not completely separable from M. gluteus minimus. It i s , in serpens, a broader muscle than that described by H i l l (1937) or Howell (1926) taking origin from the spines of the third and fourth sacral vertebrae and inserting on the greater trochanter caudomediad to the f 54. gluteal muscles. M. gluteus minimus- is situated under M. gluteus medius and i s similar in shape to the latter. It arises from the i l i a c ridge and inserts on the greater trochanter beneath M. gluteus medius. Quadriceps femorls group- analogous to the triceps group of the brachium. The muscles of this group are supplied by the femoral nerve. K. rectus femoris- has only one head in serpens and this, arises from the area close to the acetabulum and from the end of the i l i a c ridge and inserts on the patella. In townsendi and longicauda two distinct heads situated very close together were found. M. vastus lateralis- is a very heavy muscle which covers most of the lateral surface of the leg. It arises from the greater trochanter and lateral ridge of the femur and inserts on the patella, M. vastus mediaiis and M. vastus intermedialis- are very closely associat-ed but are separable. M. fastus mediaiis arises from the cranial and medial (distal position) borders of the thigh. M. vastus intermedialis arises from the entire extensor surface of the shaft of the femur. They insert together on the patella. M. sartorius- was not found. Tibial extensor group- analogous to radial and long extensors of the fore-arm, although represented by only three muscles which are supplied by the deep peroneal nerve. M. extensor digitorum longus- arises from the lateral condyl of the femur 5 5 . (near the patellar groove) and then becomes divided into four tendons which pass under the ligamentum transversum, and inserts on the terminal phalanges of digits two to five. This muscle i s sometimes called M. extensor digitorum communis. M. extensor hallucis longus- arises from the interosseous membrane between the t i b i a and fibula and from the fascia of adjacent muscles and inserts onto the terminal phalanx of the hallux, M. t i b i a l i s anterior- is a very large and heavy muscle that arises from the proximal half of the body of the t i b i a , the head of the t i b i a , and from aponeuratic fibres from region of the knee and inserts onto the f i r s t cuneiform. A secondary insertion of the f i r s t metatarsal may occur but the fibres were very indistinct. Peroneal group- muscular branches of this group are associated with both the deep and the superficial branches of the common peroneal nerve. The peroneal muscles appear to be analogous to M. extensor carpi ulnaris, M. extensor indicis and p o l l i c i s and M. extensor d i g i t ! quint! of the fore-arm. U. peroneus longus- originates on the latero-craniad process of head of the fibula, passes between t i b i a l i s anticus and peronei quart! and quint! and through a groove in the lateral malleolus to insert onto the f i r s t cuneiform and f i r s t metatarsal bones. M. peroneus brevis- arises from the cranial surface of the fibula and from the interosseous membrane, a tendon passes behind and under the lateral malleolus and inserts into a tuberosity of the f i f t h metatarsal. 56. Mm. extensores brevis- i s described by H i l l (1935) as consisting, in Thomomys, of four muscles but only three were found in Microtus. M. p e r o n e u 3 digit i quarti- arises from the head of the fibula, passes with U. peroneus longus, M. peroneus brevis, and M. peroneus digiti quinti under the lateral malleolus and becomes inserted on the metatarsal of the fourth digit. M. peroneus digit i quinti- arises from the latero-caudad head of the fibula and inserts into the distal end of the fifth metatarsal. M. extensor digitorum brevis- arises from the dorsal surface of the cal-caneus and inserts onto the second and third digits but the place of insertion was very indistinct in the specimens used. 57. B./ Flexor System Adductor group- innervated by the obturator nerve. Functionally these muscles may be compared with the pectoral group of the fore-limb whether or not they'are seriously homologous. M. gracilis- is a wide, heavy muscle which is divisible into caudal and cranial portions. The caudal part arises from the pubis caudal to the obturator foramen and inserts on the anterior surface of the t ibia. The cranial part arises from the border of the pubis deep to U.adductor longus and inserts with the caudal portion. The same two heads were found in a l l of the species. M. adductor brevis- i s the middle muscle in the adductor group. It arises from the ventral margin of the pubis ventral to the obturator foramen and cranial to the symphysis pubis and inserts on the medial (flexor) surface of the femur between M. adductor magnus and M. adductor longus. M. adductor magnus- arises from the region of the symphysis pubis and inserts on the medial surface of the femur immediately distal to M. adductor brevis. M. adductor minimus- was not found. Possibly i t is joined to M. adductor magnus in this genus. M. adductor magnus proprius- arises from the area between the obturator foramen and the ventral tuberosity of the ischium and inserts on the lateral ridge of the femur. M. obturator externus- arises from the lateral border of the obturator foramen and inserts on the trochanteric fossa of the femur. This muscle 58. was very hard to digcarn in a l l of the species. Ischiotrochanteric group- supplied by branches of the t ibial nerve. M. obturator internus- takes origin from the medial surface of the ischium and inserts on the trochanteric crest and fossa. M. gemellus superior- is rather poorly defined in serpens but takes origin in the area of the spina and dorsal border of the ischium and inserts in the trochanteric fossa in company with a tendon of M. obturator ezternus. M. gemellus inferior- arises from the cranial surface of the ischial tuberosity and inserts onto the tendon of obturator internus. This muscle is not divided into dorsal and ventral parts in serpens. M. quadratus femoris- originates on the lateral surface of the ischium and inserts onto the caudal surface of the femur and perhaps onto the lesser trochanter. Hamstring group- innervated by a special branch of the t ibial nerve. This group, is analogous to biceps brachii and forms the largest muscle group in the body. M. caudofemoralis- arises from the transverse processes of the first two caudal vertebrae and inserts onto the flexor (lateral) surface of the femur, proximal to the condyles and onto the patella. U. semimenbranosus- originates from the caudal border of the ischium and inserts, by a tendon, on the mediocranial surface of the t ibia. It is a very large muscle which is easily identified from either the lateral or medial surfaces. 59. M. semitendinous- is a fan-shaped muscle which arises from the spines of the last sacral and first two caudal vertebrae and inserts into the medial surface of the tibia with gracilis. U. biceps femoris- has only one head in the species of Microtus studied and this arises from the dorsal ischial tuberosity and inserts onto the distal end of the femur, the lateral surface of the head of the tibia and the dense fascia of the leg and tendo caloaneum. Flexor group of the leg- supplied by the t ibial nerve, except M. auadratus plantae which i 3 innervated by the lateral plantar nerve, a branch of the t i b i a l . M. gastrocnemius- i s divided into lateral and medial heads. The lateral head arises from the lateral epicondyl of the femur and from a sesamoid bone in the tendon. The medial head arises from the medial epicondyl and from a sesamoid in the tendon. Both heads insert into the tuberosity of the calcaneus. M. plantaris- with the gastrocnemius muscles belongs to the superficial layer of muscles. It arises from the lateral epicondyl by a tendon which passes over the heel beyond the tendon calcaneus, and divides into at least two tendons which are inserted, by slips, on the second phalanges of a l l the digits. M. soleus- is a spindle -shaped muscle arising from the head of the fibula and inserting on the calcaneum in company with the gastrocnemius muscles. M. popliteus- arises from the lateral epicondyl of the femur and inserts on the proximal third of the medial surface of the t ibia . 60 M. flexor digitorum t i b i a l i s - arises from the medial ridge of the tibia and is associated with M. popliteus. It becomes tendinous and passes through a groove in the medial malleolus at which point the tendon divides into two parts. One portion inserts on the plantar surface of the first metatarsal and the other part unites with the tendon of 11. flexor digitor-um fibularis. M. t ibial is posterior- is closely associated morphologically with M. flexor t i b i a l i s . It arises with the latter and, becoming tendinous, passes over the medial part of the ankle to insert on a sesamoid bone of the tarsus. M. flexor digitorum fibularis- (flexor digitorum longus) is a large muscle which occupies the space between tibia and fibula and passes be-tween the astrogalus and ankle bones to sole of foot. It arises from the tibia, and fibula and from the interosseous membrane and inserts onto a l l five digits in close association with M. flexor digitorum t i b i a l i s . M. auadrax-us plantae- not discovered. - 61 -Discussion In the course of this study a detailed comparison has been made of the muscular and skeletal anatomy of three species of Micrptines, Microtus t r ^ v ^ serpens, M. townsendi and M. longicauda. In addition some comparisons have been drawn between the foregoing species 'and M^ richardsoni as represented by a single juvenile specimen* Most, i f not a l l , species of Microtus burrow to a greater or less degree but M."serpens differs from the other three species studied in being primarily subterranean in i t s habits. On the basis of this study i t is possible to state that there seems to be very l i t t l e variation of an interspecific nature in the mus-cular anatomy of species within the genus Microtus. M. townsendi and longicauda do have some minor divergencies such as the robust t a i l muscles, the relatively much longer osicoxae, the presence of eyelashes, and the abssnce of a pollex in longicauda. Also in the latter species the cervical vertebrae are more flattened and the spines and zygopophyses show a greater degree of development. The exception to this condition is to be seen in M.4-serpens in which certain parts of the animal are apparently strongly modified in a direct-ion which seems to better adapt this animal to i t s subterranean mode of l i f e . Superficially, M. serpens is a small Microtine at least one-third smaller in mass than adults of M.townsendi and M.longicauda. External features which are of adaptive significance are the soft, plush-like pelage, the short t a i l , and the arrangement of the vibrissae, the longest. ones being furthest from the snout, rather than scattered. Of probablpf adaptive significance are the ears which are furred on the inner surface and provided with a pronounced fold on the anterior part of the helix. • - 62 -The eyes of'serpents- are only about one-half the size of those of the larger species, the same relationship ,is seen i n the lachrymal glands. Associated with the smaller eyes i s an interesting modification of the orbicularis oculi muscle whereby this becomes strongly developed, doubtless as an aid in preventing dirt from entering the eyes. Unlike most genera of burrowing mammals M.serpens does not have con-spicuously enlarged or elongated claws and fore-feet. However, many other modifications of the fore-limbs for digging occur. Conspicuous among these i s the shortening of the limbs and the inclusion of a larger part of them within the body skin. Almost the entire musculature of the anterior segment of the body i s much more strongly developed in serpens- then in any of the \ other species and i t is logical to associate this development with increased ab i l i t y in excavating burrows. This specialization of the musculature invol-ves most prominently the muscles of the maatteatery shoulder, chest and fore-limb but to a lesser degree certain of the masticatory muscles and the musculature of the spinal column are also involved. A detailed examination of these muscular specializations reveals that the masticatory muscles, i.e., masseter, temporalis, pterygoideas externus and pterygoideus internus, show some specific variations. The superficial part of the masseter muscle contains more muscle fibres in sasfrena than i t does in townsendi or longicauda; the fascia which covers the muscle i s also heavier in serpens than in the latter forms but that found in richardsoni i s decidedly the heaviest, possibly because i t is to be a larger animal. This relationship i s also seen in the development of the cutaneous maximus and platysma. Other parts of the masseter are more uniform, with serpens having muscles which appear to be slightly the largest. The temporalis muscle' i s not as extensive in serpens as i t i s i n townsendi but i t is a more robust muscle in the former. There are no differences between the external pterygoid muscles but the internal pterygoid i s decidedly larger in serpens. Few skeletal modifications corresponding to these variations in mast-icatory musculature have occurred but a small process from which part of the masseter superficialis originates has developed ventral to the cranio-ventral border of the infraorbital foramen; there i s no definite evidence on the skull of greater development of the temporalis muscles in any of the species although the posterior divergence of the zygomatic arch may be a t r i f l e broader in townsendi to accommodate the greater breadth of the temp-oralis muscle in this species. The angular process of the mandible into which the enlarged pterygoideus internus i s inserted i s , i f anything, less thickened in .serpens which i s hard to explain since the muscle enlargement was uniformly present in three specimens. The neck muscles show some specific variations, the mylohyoideus i s similar in the three species but the transversus mandibularis contains more fibres in townsendi than are found i n this muscle in jssrpens or longicauda. The digastricus is better developed in sarnens being decidedly stouter when comparisons of length to breadth are taken. In each of the species i t is a single muscle with a constriction between the anterior and the posterior bellies; although this constriction i s barely detectable in richardsoni. Both the elevators and the depressors of the lower jaw are thus seen to show their greatest development in the subterranean form. The platysma is best developed i n seggstts, the post-auricularis portion of the muscle i s more distinctly separable! from the playysma in serpens and the platysma generally has' more muscle fibres in the la t t e r . - 64 -In townsendi and longicauda i t is d i f f i c u l t to trace because the fibres adhere very closely to the skin. This same tendency i s found in such muscles as the spinotrapezius and the cutaneous maximus. A l l of the super-f i c i a l head muscles are more definitely defined in serpens than they are in townsendi and longicauda. The occipitalis muscle is separated from the platysma with d i f f i c u l t y in townsendi but i t i s easily discovered i n serpens. The interscutularis i s always a thin muscle in Microtines but i t i s appar-ently missing in longicauda although this may have been peculiar to the one specimen. The preauricularis (frontalis) adheres to the skin more t i g h t l y in townsendi and longicauda and hence is more d i f f i c u l t to trace. Orbicularis oculi i s much less distinct i n townsendi and longicauda than in serpens and the orbicularis oris i a devisible into distinct slips in serpens only. The general enlargement of the superficial head muscles may not be related to environment but the greater development of the.muscles around the eyes and mouth are undoubtedly an advantage to an underground form. These variations have not affected the skull formation i n any case. The muscles of the hyoid constrictor group do not show any marked var-iations. In the trapezius group the origin of the sternomastoideus i s found on the edge of the clavicle at i t s junction with the manubrium stemi in townsendi and longicauda rather than from the lateral surface of the manu-brium as was found in three specimens of serpens. There i s l i t t l e variation between the cleidomastoideus and cleidooccipitalis muscles. The acromioiirap-ezius muscle is decidedly different i n serpens having an insertion which i s so heavy that i t forms a thick ridge on either side of the mid-dorsal line giving the impression of a raised portion. In comparison the muscle appears to be f l a t in townsendi and is even less well-developed in longicauda. In 65. richardsoni the fixative used had had a drying effect on the muscles which may have accounted for the heavy appearance of them. In any case serpens has much the largest acromiotrapezius muscle of any of the species • which were dissected. The spinotrapezius muscle i s unique in serpens in that i t i s divisible into two distinct parts, in townsendi the anterior portion i s very limited and in longicauda and richardsoni i t i s a single muscle. The trapezius muscle draws the scapulae dorsally, upward and for-ward and the greater development in serpens of these muscles would give i t greater strength and f l e x i b i l i t y . Correlation between this muscle develop-ment and the structures of the scapulae seems to be impossible as far as shape i s concerned although the vertebral border of the scapula i s sl i g h t l y more thickened in serpens. In the dorsal division of the myotomic musculature there seem to be more fibres i n each division of the sacrospinalis, i l i o c o s t a l i s , and longissimus muscles of serpens. The muscles which are attached to the axis, the semispinalis capitis, spinalis dorsi and rectus capitis posterior major are slightly better developed i n serpens although the difference i s not sufficient to account for the development of transverse processes in serpens instead of merely a slight elevation on their sites as i s seen i n townsendi and longicauda. The other body muscles do not show sufficient differences for conclusions ,of any kind to be drawn from them with the exception of the t a i l muscles which are decidedly reduced in serpens. They are much better developed in longicauda than in either of the other species. . The muscles attached to the pectoral girdle show the most marked differences of any of the muscles. They are almost universally better 66. developed in serpens than they are i n townsendi ear longicauda. The levator scapulae and serratus anterior muscles are easily divisible into separate muscles in serpens but this is not as clearly the case in townsendi or longicauda where they are separable with d i f f i c u l t y , i f at a l l . Possibly since these muscles are used to draw the scapulae craniad, ventrad, and against the thoracic wall and have become enlarged to perform this function they have become two separate muscles in the course of development. The rhomboideus muscle i s actually smaller i n townsendi than i t i s in serpens which means that i t i s a great deal smaller when considered relatively, i t s function i s to draw the scapula toward the vertebral column. The latissimus dorsi i a inserted into the a x i l l a by widely separated digitations in town- sendi , in serpens and longicauda the insertion i s single. The aerfcmfco-deltoideus i s very prominent in serpens being so well developed that i t alters the appearance of the position of the fore-limbs giving them the appearance of being drawn craniad. The splnodeltoideus i s also larger in serpens. The last two muscles raise and rotate the humerus and greater development must be a decided help i n digging. The pectoralis muscles are the most remarkable of the muscles attached to the pectoral girdle and limb. When dissected from the specimen they were found to have a volume which was almost exactly the same in serpens and townsendi although the latter had a total body volume of almost one and one-third times that of the former. This greater development is correlated with the development of a keel on the manubrium sterni and the sternebrae in serpens to give a greater area for muscle attachment. Since these muscles serve to draw the fore-limb toward the chest the enlargement found i n serpens i s in accordance with the enlargement of the extensor muscles found 67. in the same animal. The fore-limb is shortened in serpens in per cent of total body length and hence the muscles are also shortened. The muscles of the brachium are not as clearly defined (readily distinguishable) in serpens but are heavier muscles when proportion of length to width i s considered. The muscles of the anti-brachium give the appearance of being even smaller than they really are owing to the differences in sizes of the species. Possibly the muscles are well-developed as a compensatory measure for the shortening. The muscles of the back and those attached to the pelvic girdle and limb show practically no specific differences. Apparently the hind limbs do not become especially adapted to a subterranean existence since they are not required for digging, o. Microtis7serpens appears to represent one of the earliest stages of adaptive specialization of a mammal for a subterranean l i f e . In this species the fur, eyes, ears, and t a i l a l l show approach to the condition found i n such highly specialized burrowers as the moles. The musculature reveals many obvious changes in mass and some subdivisionX^but other than a slight lengthening of the scapula and a fore-shortening of the humerus and radius no radical skeletal changes have occurred. The position of the fore-limb has not been modified and the fore feet and claws are not enlarged. E X T E R N A L F E A T U R E S Right Fore foot • Left Hind Foot 'Palmar Surface Fig. 1 VERTEBRAE Atlas Axis C e r v i c a l ( 3 r d ) serpens longicauda T h o r a c i c dorsal arcii Tra ns verse, p rocess wing sp i nous process articular proiif.s'j vertebral fonamen transverse process centrum. urTiCular process transi/er-se process Iransk/erse foramen dorsal spine diapopnysis articular process ana pophysis ventral tuben:!i Fig. 2 • r S E C O N D L U M B A R i e t o puphys-s diapophysis spinous process body. .napophys irtopophjsis L e f t i ^ a t e r a t S u r f a c e D o r s a l S u r f a c e anterior" articular n-act*ss transverse pre meta poph^ cis/ p o s t e r i o r -S^p'K^cs process rod i mentary. articular process t r a n s v e r s e process" CAUDAL rr-i t -S A C E U M (Anterior P o r t i o n COMPARISON OF CAUDAL VEKTEBEAE. Serpens townsendi longicauda VERTEBRAE. A F i g . 3 S T E E N U . M ^ 1 IF _manu tori urn . I** costal cartilage _lo?e(ed surface sternetara uphi'sTet-nurn s e r p e n s 4~L _ f l o t surface T o w n s e n d i ' F i g . 4 w t SKULL Nasal is Pterygoid Fissure « Foramen Magnum 14 Foramen Ovale i s Tympanic Bulla 16 Occipital Condyle 17 External Dorsal Surface Auditory Meatus 18 Occipital l Premaxilla 3 Infra-orbital Fissure 4 Maxillary 5 Malar Process. 6 Frontal 7 Zygomatic 8 Sguamosal 9 Parietal 10 Interparietal 11 Lambdoidal Crest L e f t L a t e r a l View Fig. 5 SKULL 14 Temporal Ridge is Exoccipital i«. Mastoid i7 External Auditory Meatus is Para occipital Process is Basioccip'ital zo F o r a m e n Magnum Ventral Surface 1 Premaxilla 2 [ncisive Foramen 3 Palatine 4 Pre-sphenoid j Ali-sphenoid 6 Para pterygoid 7 Mesopterygoid B Basi-sphenoid s Auditory Bulla io Styloglossus process II Supraoccipital it Lambdoidal Crest 13 Zygomatic Arch. Occipital Surface Fig. 6 L MANDIBLE " 1 Media l Aspect i n a s s e i e n t f l d g e L a t e r a l A s p e c t Fig. 7 LEFT SCAPULA 1 . -Subscapular ?osso ..Subscapular 7?ida< Medial Aspect Anterior "Border. •Supraqlenoid. T u b e r o s i t y C o r a c o i d . "Process A c r o m i o n P r o c e s s — M t t a c r o m i o n R e c e s s 1 I n f r a t j k n o i d . T u b e r c l e ' A * i l l a r i j Borti«r .' -Supraspinous ToSSi • •Infraspinous F o s s a -- F o s s a for <er«s JVJaj Lateral Aspect L . J Fig. 8 /• HUMERUS Greater tuberc/e-heacf Ned/a/ candy* LyjTera/ condyl*. Larera/ / Z / A - j ^-ft—Medial epicorrdy/a. eptcondyk VjLJ^K. Trvchk O Flexor Surface Extensor Surface Fig. 9 R A D I U S AND ULNA 6*rmi-lunur notch iOronoid process &X'ad of rudii O l e c r a n o n p r o c e s s c o r o n o i d p r o c e s s -— lateral Wdge - olecranon -semi-funar notch - interosseous crest -styloid Lateral A s p e c t Fig. 10 F OS COXAE Posterior Superior Spine Gluteal Notch Superior Borde> Articular Surface- of Acetobuk Sciohc NotcJt-Body of tschi Tuberosity of Ischium Crest of the Ilia Lateral Ridoe •Inferior Border Super/or Central Spine lleo- Pectineal Eminence Ascending &omus of Pubis Angle of Pubis Inferior Tuberosity Descendirry Pomus of Pub,s Lateral Aspect (Sight) Fig. 12 TIBIOFIBULA r F i g . 13 Vr a -u -Tr -N -H Q -C e -C o -Lm-G m-F o -i s t a n d H a n d radius triangular navicular hamate centrale capitate lesser multangulum greater multangulum folc. •fa r m i s metacarpal m era truss. I A n k l e a n d F o o t C - colcaneus Ta - talus Ti - ti biale Na- navicular Cd- cuboid C, - 1^  cuneiform C2- S^cuneifor-m Cj ~ 3^ * cuneiform rnt - metatarsal 3 . E x T E N S O E S u E F A C E OF T H E . RIGHT MAN US AND P E S  F i g . 14 HEAD M U S C L E S r ont a lis Dor so I View FACIAL MUSCLES Dilator noris Platysma Lateral Vieuj Lateralis profundus Fig. 15 VENTRAL MUSCLES or NLCK Transversus_ mandtbularis. 'Sternohyaideus.-O r n o V r y o i d e u s Sternomastoideus. Cleidomastoideus Clavotrapezius M y^lohjoide . . . . i l l , Sternothyroideus i«/ til Longus colli '>V '| •, "j|L Rectus capitis' \^.\f-i;/ § Scalenus DEEP MUSCLES or THORACIC REGION Pectoralis_ Intercostales interni Tntercostales externi Rectus abdominis^  *\ Serratus magnus , spinalis dorsi 1 ierratus posterior Fig. 16 DEEP BACK MUSCLES 3 d o r s i L u m b a r R e c f l e x o r c a u d a e l a t e r a l i s E x t e n s o r c a u d a e m e d i a i i s E x t e n s o r c a u d o e l a t e r a l i s C a u d o J R e g i o n Fig. 17 MU5CL.ES ATTACHED TO PECTORAL GIRDLE O c c i p i t a l i s . L e v a t o r a u n s -l o n g u s . A c r o m i o -t r a p e z i u s -— S p l e n i u s C l a v o i r a p e z i u s A c r o t n i o -d e l t o i d e u s S p i n o d e l l o i d e u s . _ a t i s s i m u s d o r s i _ 3 p i n o t r a p e z i u s S t e r n o i n a s t o i d e u s M Ii P e c t o r a l i s i r t i n o r D o r s a l M u s c l e s . C l a v o t r a p e z i u s A c r o m i o -d e l t o i d e u s P e c t o r a l i s m a j o r V e n t r a l M u s c l e s Fig. 1 8 r MUSCLES OF S H O U L D E R AND U P P E E A 2 M cof'acobrac.hialrs-biceps broch subscapulars do rsoe pi t ro ch 1 ea r i triceps brachii rhombaideus mii-ioi brachial accom iotrupezrus rhomiiotdeus major major ateral Sur face Fig. 19 r LATERAL MUSCLES OF H I P Gluteus AND HIND, LIMB Semitendinosus iceps f e m o r i s Tensor fasciae-lo tae Vastus lateralis MEDIAL MUSCLES- OF HIND L Vastus lateral; Rectus fe Vastus Adductor brevi: Adductor ma Gracilis soas major neus ctor longus membranosus endinosus Fig. 20 Literature Cited Edgeworth 1916 - Cranial Muscles of Vertebrates Cambridge University Press. 1911 - Morphology of Cranial Muscles in some Vertebrates Quart. Jour. Micro. S c i . , 56: 167-316, 100 figs, in text. Greene, E.C. 1935 - Anatomy of the Rat American Philosophical Society Transactions University of Pennsylvania Press, 1-370, 339 figs, in text. Gregory W.K. and Camp, C.L. - see p. 2. ! ' H i l l , J . E . 1937 - Morphology of the Pocket Gopher. Mammalian Genus Thomomys Univ. of Calif. Publications in Zoology, 42: 81-172, 26 figs, in text. Howell, A .B. 1924 - Individual and Age Variations in Microtus Mont anus eSa>3emite Jour. Agric. Research _28, No. 10, Dec 4: 997-1015. 1926 - Anatomy of the Wood Rat Williams and Wilkins Co., Baltimore i-x, 1-225, pis. 1-3, 37 figs, in text. 1936 - Phylogeny of the Distal Musculature*, of the Pectoral Girdle Jour, of Morphology 60: 287-315, 3 figs, in text. 1936 - The Phylogenetic Arrangement of the Muscular System Anat. Record 66: 295.316. Lewis, W.K. 1910 - The Development of the Muscular System ' In Keibel and Mall, Manual of Human Embryology 1-ch. 12: 454-522, 55 figs, in text. McMurrich, J.P. 1902(b) Phylogeny of the fore-arm flexors Amer. Jour, of Anat. 2: 177-209 (mammals 166-209) Parsons, F.0. 1894(b) On the Morphology of TendonAchill&'s v Jour. Anat. Physiol. 26i 414-418, 2 figs, in text. Gregory, B.K. and Camp, C.L. 1918 - Studies of Comparative Myology and Osteology Bull . Amer. Mus. Nat. Hist. 38: 447-563, pis. 39-50, 16 figs, in text. 

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