266 University of California PuMioations in Zoology ■ [Vol. 21 



lig-lit aniline black. Nose may or may not be darker than area in front 

 of eyes. Ears large,' not concealed by fur, the anterior border heavily 

 furred with hairs of same color as light tipped hairs of upperparts. 

 Tail distinctl}^ bicolor, vary'ing from mars brown to aniline black above 

 and pallid neutral gray below, though when tail is scantily haired 

 the yellowish scales show through. Hands and feet covered with pallid 

 or light mouse gray hairs. In worn pelage : upperparts much duller, 

 more brownish, occasionally with a marked suffusion of cinnamon 

 colored, light-tipped hairs and with a reduction in number of long, 

 dark overhairs. 



Skull. — As compared with that of Micro t us montanus montanus, 

 its general form is broader, relatively larger in size, with dorsal surface 

 well marked by ridges for attachment of muscles, and with zygomatic 

 arches more widelj^ spreading. Dorsal profile essentially the same 

 as in montanus, excepting interorbital region which is less depressed. 

 Depth of brain case relatively deeper. Occiput abruptly truncate, 

 permitting the condyles to be visible when skull is viewed from above. 



Brain case roughly hexagonal, rounded at sides and truncated 

 anteriorly by conspicuoasly developed postorbital tuberosities. The 

 postorbital processes are variable in their development, depending on 

 age; in sexually adult individuals usually conspicuously developed, 

 becoming prominent and spread out laterally in very old individuals. 

 Longitudinal ridges along outer edges of parietals enclosing a 

 hexagonal area and more prominent than in old individuals of mon- 

 tanus. Interorbital constriction narrow. Interorbital ridges coalesced 

 in old individuals, forming median ridge and obliterating median 

 sulcus. Lambdoidal ridge absent superiorly and represented by outer 

 extremities only. Interparietal variable in appearance. In some 

 individuals anteroposterior diameter, exclusive of median projection, 

 is greater than one-half transverse diameter, in others nearly one-third 

 transverse diameter. Posterior border convex or biconvex, with lateral 

 extremities variable. Frontals narrow, either tinmcate or emarginate 

 posteriorly. 



Rostrum essentially as in montanus, usually deeper proxinially, 

 and with least depth behind incisors equal to or greater than width 

 in same region. Nasals moderate in length, slender, slightly con- 

 stricted anterior to middle, the widest portion of nasals less than 

 transverse diameter of incisors at base. Posterior terminations of 

 nasals variable, either truncate or emarginate, usually on a line with 

 ends of premaxillae or slight!}^ anterior to them and in most cases not 

 extending as far as plane of incisors. Transverse diameter of anterior 

 narial opening equal to or greater than vertical. Anteorbital foramen 

 very wide, and relatively shallower, with considerable variation in its 

 outlines. Plate bounding outer wall superiorly incomplete. 



Occiput when viewed from behind not differing in any visible 

 character from that of montanus, the median depth equal to about 

 57 per cent of greatest width across lambdoidal ridge of brain case. 

 Foramen magnum usually roughly triangular, its corners rounded, 

 with vertical diameter usually greater than transverse. Paroecipital 

 processes variable, usually directed more backward tlian downward, 

 their extremities usuallj^ not closely applied to bullae. Posterior 

 margins of paroccipitals continued upward as more conspicuous 



