The remaining forms from the Bonneville Basin, namely, _Thomomys bottae sevieri_, _convexus_, _tivius_ and _stansburyi_ are all easily distinguished from _robustus_. Specimens of _sevieri_ are paler, smaller in every measurement taken, and the skulls are weaker and less angular. All specimens of _convexus_ are paler, the skulls are more convex dorsally and narrower, with less ridging and angularity. Both _tivius_ and _stansburyi_ are small dark forms, with weak, smooth, small skulls as compared with _robustus_ which is light colored and has compact, ridged and angular skulls. _Remarks._--Twenty-three specimens were obtained at a small isolated spring. Critical study of animals taken only a few miles to the east prove them to be so different as to be referable to another subspecies, _albicaudatus_. _T. b. robustus_ is an endemic form in this desert valley. The variable color is noteworthy but difficult to explain in an isolated population as small as this one. All five of the gray animals are females of which four are lactating adults. The affinities of this subspecies are with _albicaudatus_ to the east, but enough time has elapsed since isolation to enable them to differentiate. _Specimens examined._--Total, 23, from the type locality. =Thomomys bottae minimus= Durrant _Thomomys bottae minimus_ Durrant, Proc. Biol. Soc. Washington, 52:161, October 11, 1939; Marshall, Journ. Mamm., 21:154, May 14, 1940. _Type._--Male, adult, skin and skull, No. 263942, U. S. National Museum (Biological Surveys Collection); Stansbury Island, Great Salt Lake, Tooele County, Utah; June 25, 1938; collected by William H. Marshall; original number 141. _Range._--Known only from the type locality. _Diagnosis._--Size small (see measurements); tail relatively long. Color: Upper parts Pinkish Buff, darker on head; underparts Pale Pinkish Buff; front and hind feet white; nose, chin and postauricular patches black. Skull: Long, slender and nearly devoid of ridges; braincase moderately inflated; interparietal quadrangular; zygomatic arches weak, widest in temporal region, but neither widely spreading nor angular; nasals straight and truncate posteriorly; extension of premaxillae posterior to nasals relatively great; tympanic bullae moderately inflated; palatal pits deep; rostrum short but narrow; interpterygoid space moderately lyre-shaped; upper incisors narrow; molars light.