_Diagnosis._--Size medium (see measurements). Color: Upper parts Dresden Brown, grading over sides to Pale Buff on underparts; chin white; ears small, pointed, with deeply pigmented pinnae; postauricular patches grayish black; nose dusky. Skull: Long and narrow; nasals long, rounded proximally and usually simple distally; rostrum long; interparietal triangular; tympanic bullae large, and well inflated ventrally; basioccipital narrow; palate narrow; palatal pits shallow; dentition light. _Comparisons._--Near topotypes of _fossor_ can be distinguished from topotypes of _Thomomys talpoides ocius_ as follows: Size larger throughout. Color: Darker throughout, being dark brown as opposed to grayish. Skull: Longer and narrower; nasals and rostrum longer; extension of supraoccipital posterior to lambdoidal suture markedly less; tympanic bullae markedly smaller; upper incisors longer and not as strongly recurved. Among the races of _Thomomys talpoides_ occurring in Utah, _fossor_ most closely resembles _Thomomys talpoides uinta_ in color and size, but differs from it as follows: Ears smaller, more pointed and with more darkly pigmented pinnae. Skull: Longer, narrower and weaker; rostrum longer; nasals longer, and rounded proximally as opposed to markedly emarginate; interparietal triangular instead of roughly pentagonal; tympanic bullae larger and more inflated ventrally; basioccipital narrower; palate narrower, palatal pits shallower; dentition lighter. _Remarks._--Bailey (1915:111) remarked that _fossor_ was one form that held its distinctive characters over a wide range. At that time, its range was understood to include practically all of the mountainous parts of Colorado, Utah as far west as the central part of the state, and parts of New Mexico, Arizona and Wyoming. Subsequently three new forms have been named from central Utah, (Goldman 1938:334-337) thereby showing variation to be much more prevalent than formerly supposed. The range of _fossor_ in Utah, as now understood, is limited to the mountainous parts of the state south and east of the Colorado and Green rivers in Grand and San Juan counties. The Utah specimens are not typical. At first glance some differences are noted in the premaxillae and nasals. Four specimens in the collections of the Museum of Natural History, University of Kansas, three from 3 miles east of Creede, Mineral County, and one from 10 miles east of Lake City, Hinsdale County, Colorado, both of which lie north and east of the type locality of _fossor_ show the same characters as the Utah specimens. Eight specimens from Oak Spring are intergrades between _fossor_ and _ocius_. In size and color they are like _fossor_, but the skulls are intermediate. Because the animals are more like _fossor_ in the majority of characters, they are here referred to that race.