_Additional records._--_Sanpete County_: Ephraim, 5 (see Goldman, 1938:336).
_Thomomys fossor_ Allen, Bull. Amer. Mus. Nat. Hist., 5:51, April 28, 1893; Bailey, N. Amer. Fauna, 39:111, November 15, 1915; Barnes, Bull. Univ. Utah, 12 (No. 15):85, April, 1922; Bull. Univ. Utah, 17 (No. 12):102, June, 1927; Hall, Univ. California Publ. Zoöl., 37:4, April 10, 1931.
_Thomomys talpoides fossor_ Goldman, Journ. Mamm., 20:234, May 14, 1939.
_Type._--Male, adult, skin and skull, No. 5240/4120, American Museum of Natural History; Florida, 7,200 ft., La Plata County, Colorado; June 25, 1892; collected by Charles P. Rowley (after Allen, type not seen).
_Range._--In the mountains of San Juan and Grand counties, east of the Colorado and Green rivers.
_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.