290 University of California PvMicatioiis in Zoology [V^ol. 21 



bui? tipped hairs. Nose and forehead strongly mixed with dark over- 

 hairs. Ears small, almost concealed by fur, with anterior border con- 

 cealed by blackish slate hair bases, while posterior border is covered 

 with hairs of same shade as light tipped hairs of upperparts. Tail 

 distinctly bicolor, aniline black above and pallid neutral gray below, 

 though when tail is scantily haired the yellowish scales show through. 

 Hands and feet covered with dull white hairs. In worn pelage : gen- 

 eral coloration as a whole much duller or more grayish. The blackish 

 slate hair bases enter into general coloration of upperparts to a larger 

 extent. 



Skull. — In fully adult individuals the skull attains a length equal 

 to that of Microtus californicus niariposae, but width of zygomata 

 rarely exceeds this dimension in the latter, so that the general form 

 is somewhat less robust and less angular. Dorsal and ventral profiles 

 essentially the same as in mariposae except that nasals are usually 

 less abruptly depressed anteriorly and depth of brain ease is slightly 

 shallower. Occiput rather obliquely truncate, concealing part of 

 occipital condyles, when skull is viewed from above. (See plate 8.) 



The majority of skulls have a rather long brain case, as compared 

 with anterior portion of skull, subovate in outline, but with moderately 

 developed postorbital tuberosities that exhibit a tendency to spread 

 out laterally along posterior border of orbit, though they are suf- 

 ficiently prominent to give the anterior border of brain case a squarish 

 appearance. Surface of brain case, in mature individuals, with well 

 indicated longitudinal ridges along outer edges of parietals. The inter- 

 orbital region is very broad, more so than in other races of this group. 

 Interorbital ridges usually poorly defined, but normally with distinct 

 sulcus between the two. Interparietal variable in outline, usually sub- 

 rectangular, its posterior border decidedly convex, its lateral extrem- 

 ities rather obliquely truncate, and with median projection of normal 

 size. Frontals large, truncate, or emarginate posteriorly. 



Preorbital region long and slender, with gradually tapering ros- 

 trum, slightly wider than greatest breadth of interorbital constriction 

 and rather shallow proximally; least depth behind incisors slightly 

 greater than width in same region. Nasals not projecting beyond 

 incisors, usually terminating posteriorly on a level with nasal branches 

 of premaxillaries, somewhat constricted a little anterior to middle, 

 attenuate posteriorly but rarelj' as noticeably as in angusticeps. An- 

 terior narial opening with transverse diameter about equal to vertical. 

 Anteorbital foramen rather shallow and wide, with superior breadth 

 considerably greater and in most cases at least twice inferior breadth. 



Depth of occiput, when viewed from behind, shallower than in 

 Microtus c. niariposae, and with flattened occipital region, the median 

 depth in majority of skulls examined about 57 per cent of greatest 

 width across lambdoidal ridge of brain case. Foramen magnum 

 variable, with vertical diameter equal to or less than transverse. 

 Paroccipital processes light, directed more downward than backward 

 and rather closely applied to bullae ; their posterior margins con- 

 tinued upward as sharp crests, rarely touching the lambdoidal ridge 

 superiorly. 



