1918] Kellogg: Microtus calif ornicus Group of Meadow Mice 13 



ceous-tawny tint, varying considerably in exact shade among different 

 individuals. Basal portions of hairs blackish plumbeous ; the long 

 overhairs grading from dark Mars brown to black. Sides lighter than 

 upperparts, with the light tipped hairs approaching more closely light 

 ochraceous buff in color. Rump may or may not be brighter than rest 

 of upperparts. Underparts pallid mouse gray, irregularly darkened 

 by basal underfur showing through ; inguinal and abdominal region 

 occasionally washed with buffy; anal area usually pure white. Whis- 

 kers light colored or grayish, with basal portions slightly if at all 

 darker than terminal portions. Ears with anterior border covered 

 with hairs identical in color with rest of upperparts. Tail distinctly 

 bicolor, above dark fuscous black, below pallid mouse gray, though 

 tail is usually so scantily haired that the yellowish scales show through. 

 Hands and feet covered with hairs of a French gray tone. In worn 

 pelage : Mass effect more buffy or grayish. Light tipped hairs of 

 upperparts more buft'v, in some specimens approaching ochraceous 

 buff. 



Skull. — As compared with that of calif or nicus, general form nar- 

 rower, size slightly less, dorsal surface not distinctly marked by ridges 

 for muscle attachment, and zygomatic arches not as widely spreading. 

 Dorsal profile rather evenly convex, slightly depressed interorbitally, 

 with region of greatest convexity in plane with lacrimals. Occiput 

 rounded, with condyles almost entirely visible when skull is viewed 

 from above. Skull proportionally shallower than in californicus. 



Brain-case broadly ovate or rounded, not conspicuously truncated 

 anteriorly by postorbital tuberosities. In old adults the longitudinal 

 ridges near outer edges of parietals are feebly developed or wanting. 

 Interorbital ridges are not prominent, while sulcus between them may 

 be nearly closed in old adults. Interorbital constriction narrower 

 than anterior portion of rostrum. Lambdoidal ridge absent superiorly 

 and represented by outer extremities only. Interparietal somewhat 

 rectangular but variable in appearance, with posterior border convex 

 and with outer extremities rather abruptly truncate ; anteroposterior 

 diameter of interparietal, exclusive of median projection, less than 

 one-half of greatest transverse diameter. Frontals convex or truncate 

 posteriorly. 



Rostral region elongated, gradually tapering anteriorly, the least 

 depth behind incisors being greater than, or at the least equal to, width 

 in same region. Nasals long, spatulate, extending to or beyond in- 

 cisors, with anterior declivity not as abrupt as in californicus, and 

 notched or emarginate at posterior termination. Ascending arm of 

 premaxillae extending beyond plane of nasals but not beyond that of 

 lacrimals. Opening for anterior nares wider transversely than ver- 

 tically. Anteorbital foramen similar in outline to that of californicus. 



Occiput not depressed to any appreciable extent, but median depth 

 may equal as much as 60% of greatest width across lambdoidal ridge 

 of brain-case. Foramen magnum subtriangular, with rounded angles. 

 Paroecipital processes light, directed somewhat downwards and back- 

 wards, usually not applied to bullae ; their tips extending hardly be- 

 yond most posterior point on supra-occipital above foramen magnum. 

 Posterior margins of paroccipitals continued upwards as sharply 



