From topotypes of _Thomomys bottae bonnevillei_, _sevieri_ differs in: Size smaller throughout. Skull: Smaller in every measurement taken, weaker, smoother and less angular; dentition smaller and weaker. Topotypes of _sevieri_ are easily distinguished from those of _Thomomys bottae robustus_ by smaller size, and smaller, markedly weaker skull which is less angular and ridged. Among named races of _Thomomys bottae_, _sevieri_ is closest geographically to _convexus_, but differs from it as follows: Size larger; hind foot longer. Skull: Smaller in every measurement taken; nasals shorter and not so flaring distally; rostrum weaker, narrower and not so depressed; zygomatic arches markedly weaker and less angular; lacrimal processes smaller; supraoccipital narrower and higher; paroccipital processes weaker; tympanic bullae smaller; dentition markedly weaker. Topotypical specimens of _sevieri_ can be readily distinguished from those of _Thomomys bottae tivius_ by Pinkish Buff instead of Mummy Brown on upper parts. Tympanic bullae larger and markedly more inflated; nasals longer; zygomatic and mastoidal breadths greater; rostrum longer and more depressed; upper incisors longer and wider; molariform teeth smaller. The skulls of _sevieri_ resemble those of _tivius_ more closely than those of any other subspecies. _Remarks._--The House Mountains in western Millard County are surrounded by desertlike terrain that is seemingly unsuited to pocket gophers. In these mountains, gophers were sought in vain at several localities, including Antelope Springs which superficially appeared suitable for the animals. Pocket gophers were found only at the type locality, Swasey Spring, which is well above the highest level of the Pleistocene Lake Bonneville. _T. b. sevieri_, like _T. b. minimus_ on Stansbury Island, Great Salt Lake, appears to remain only on land that was an island when Lake Bonneville was at its highest level. _Specimens examined._--Total, 10, from the type locality. =Thomomys bottae convexus= Durrant _Thomomys bottae convexus_ Durrant, Proc. Biol. Soc. Washington, 52:159, October 11, 1939. _Type._--Male, adult, skin and skull, No. 2482, Museum of Zooelogy, University of Utah; E side Clear Lake, 4,600 ft., Millard County, Utah; May 20, 1938; collected by S. D. Durrant; original number 1401. _Range._--Westcentral Utah in Delta Valley.