294 University of California Pnhlications in Zoology [Vol. 21 



ochraceous buif suffusion irregularly darkened by plumbeous hair 

 bases showing through; anal area pure white. Terminal half of 

 whiskers pallid neutral gray, basal half blackish. Upper and lower 

 lips white. Ears relatively large, not concealed b.y fur, with anterior 

 border concealed by plumbeous bases of light tipped hairs. Tail 

 bicoJor, dark mars brown above and scarcely covered with light grayish 

 hairs below, with the yellowish scales showing through. Hands and 

 feet covered with light mouse gray hairs. Young : color of upperparts 

 duller and darker than in adults, with, larger proportion of long dark 

 overhairs, though in some specimens there is a conspicuous brightening 

 due to presence of buffy hairs. 



Skull. — Although the skull of this form is lighter and shorter than 

 in typical sierrae, it is similar in general outline. The depth of brain 

 case is relatively greater, though dorsal profile is essentially the same. 

 Occiput rather obliquely truncate, permitting posterior portion of 

 condyles to be visible when skull is viewed from above. (See plate 8.) 



Compared with skulls of sierrae the brain case is rather long and 

 narrow, slightly rounded at sides, but with postorbital tuberosities 

 not sufficiently developed to produce any break in contour of skull. 

 The brain case is slightly inflated superiorly, with fairly well indicated 

 longitudinal ridges along outer edges of parietals. Interorbital region 

 very narrow, more so than in other races of this group. Interorbital 

 ridges usually coalesced in adults, forming a shallow median crest. 

 Interparietal variable in outline, its area decidedly less than that of 

 parietal. Width of interparietal at extremities contained at least three 

 and occasionally four times in its greatest length. Frontals convex 

 or emarginate posteriorly, but more elevated anteriorly than in other 

 races in this group. 



In comparison with Microtus m. nvordax, the rostrum is essentially 

 the same, though ratio of length to proximal depth is more noticeable 

 as contrasted with shallow depth of former. Least depth behind 

 incisors usuallj' greater than width in same region. Nasals at widest 

 point but slightly narrow^er than rostrum, and projecting as far as 

 or beyond plane of incisors, their posterior terminations usually 

 exceeded hy posterior margins of premaxillaries. Opening for an- 

 terior nares with transverse diameter about equal to vertical. Ante- 

 orbital foramen rather short and narrow with superior portion at 

 least twice width of inferior. 



Depth of occiput when viewed from behind more than half width 

 of brain case ; the median depth may equal as much as 61 per cent 

 of greatest width across lambdoidal ridge of brain case. Dorsal profile 

 more convex, and ratio of depth to length greater, than in mordax. 

 Foramen magnum roughly triangular, not flattened superiorly as in 

 mordax, with vertical diameter slightly exceeding the transverse. 

 Paroccipital processes light, directed more backward than downward, 

 not applied to bullae at their extremities, and their posterior margins 

 continued upward, as in mordax. The median ridge of basioccipital 

 poorly defined. Auditory bullae small and constricted, their posterior 

 boundaries more obliquely truncated than in mordax, with posterior 

 margins slightly indented by foramina. 



Palate about the same as in hemardinus, but with shallower 

 palatine sulci, broader median ridge, and more strongly developed 



