Type.—Male, adult, skin and skull, No. 18852/25586, U. S. National Museum (Biological Surveys Collection); dry sagebrush mesas at Harveys Ranch, Smiths Fork, 6 mi. SW Fort Bridger, 6,657 ft., Uinta County, Wyoming; May 24, 1890; collected by Vernon Bailey; original number 1194 (after Bailey, type not seen).
Diagnosis.—Size small (see measurements). Color: Upper parts Tilleul Buff overlaid with Avellaneous, grading over sides and flanks to nearly white on underparts; underparts with faint wash of creamy white; postauricular patches small and dusky and completely circling the ear; nose and cheeks dusky; front feet, hind feet, throat, ventral surface of tail and distal half of tail white. Skull: Small, slender but compact; nasals rounded posteriorly; extension of premaxillae posterior to nasals very short; zygomatic arches robust, but not widely spreading, widest posteriorly; interparietal large and pentagonal in shape; extension of supraoccipital posterior to lambdoidal suture long; tympanic bullae actually as well as relatively large; basioccipital narrow; pterygoid hamulae long and ridged; upper incisors short and strongly recurved.
Comparisons.—Compared with one topotype and seven near topotypes of Thomomys talpoides pygmaeus, ocius differs as follows: Size larger in every measurement taken. Color: Lighter throughout, grayish as opposed to brown; distal half of tail white as opposed to only a few white hairs at tip of tail. Skull: Larger in every measurement taken; skull more compact; zygomatic arches heavier and more widely spreading posteriorly; tympanic bullae larger; upper incisors larger, but equally strongly recurved; molariform teeth larger.
Topotypes of ocius can be distinguished from those of Thomomys talpoides uinta as follows: Color: Lighter throughout, grayish as opposed to brown. Skull: Nasals rounded posteriorly as opposed to emarginate; zygomatic arches more robust; interparietal pentagonal as opposed to subquadrangular; extension of supraoccipital posterior to lambdoidal suture markedly greater; tympanic bullae actually as well as relatively much larger; upper incisors short and strongly recurved as opposed to long and procumbent.
Specimens of this subspecies can be distinguished from all other members of the species Thomomys talpoides occurring in Utah by their grayish color, and by small, compact skulls with very large tympanic bullae and short strongly recurved upper incisors.
Remarks.—Two specimens from Vernal, Uintah County, are intergrades between ocius and uinta. They resemble uinta in size and dorsal color, but are slightly lighter tending toward the color of ocius. Ventrally they are intermediate in color but more like ocius. The skulls are more like those of ocius in general appearance, extension of supraoccipital posterior to the lambdoidal suture, shape and thickness of the zygomatic arches, posterior tongues of premaxillae, size of tympanic bullae and recurved upper incisors. They more closely resemble uinta in shape of posterior ends of nasals, basioccipital and shape of the zygomatic processes of the squamosals. In all of the above mentioned characters, they are intermediate between the two named forms, but tend towards one or the other as listed. The majority of characters are more as in ocius to which they are here referred.
When Goldman (1939:233, 234) listed the named subspecies of Thomomys talpoides, he hesitated to include ocius and merely mentioned that ocius, pygmaeus and idahoensis might also belong to talpoides. Davis (1939:240, 241) found intergradation between idahoensis and fuscus and also between idahoensis and pygmaeus, and, therefore, arranged the last two mentioned forms as subspecies of talpoides. This present study reveals intergradation between ocius and uinta, and also between ocius and fossor (see account of fossor). Therefore, ocius is properly to be treated as a subspecies of the series of intergrading forms of which talpoides is the earliest named.
All specimens of ocius known from Utah are from the extreme eastern part of the northeastern corner of the state. The type locality of ocius is near Fort Bridger, Wyoming, which is north of Utah. I have seen one specimen from 12 miles west of Linwood, Daggett County, Utah, on Henrys Fork in Wyoming. Additional collecting in northern Utah probably will reveal ocius to inhabit also parts of northern Utah.
Specimens examined.—Total, 4, distributed as follows: Uintah County: Vernal, 2 (C. M.); Uncompahgre Indian Reservation, 2 (A. M. N. H.).