Among named races of _T. bottae_, _birdseyei_ most closely resembles _trumbullensis_ in size, but differs as follows: Hind foot and tail longer. Color: Lighter throughout; postauricular patches smaller and lighter. Skull: Larger; mastoid breadth less; zygomatic arches wider and more widely spreading posteriorly; median frontal depression more marked; extension of premaxillae posterior to nasals greater; tympanic bullae less inflated ventrally; molariform teeth larger. For comparisons with _Thomomys bottae planirostris_ see account of that form. _Remarks._--_T. b. birdseyei_ is apparently endemic to the mountainous area of southwestern Utah in Washington and Iron counties. It intergrades with _virgineus_ and with _planirostris_ as described in the account of the latter. _Specimens examined._--Total, 8, distributed as follows: _Washington County_: Pine Valley, 1 (U. S. N. M.); Pine Valley Mountains, 5 mi. E Pine Valley, 8,300 ft., 3 (U. S. N. M.); Pine Valley campground, 6,800 ft., 1 (R. H.); 3/4 mi. E town of Pine Valley, 6,500 ft., 3 (R. H.). _Additional records._--_Washington County_: Hebron, 1; Mountain Meadows, 2 (Bailey 1915:75). =Thomomys bottae virgineus= Goldman _Thomomys bottae virgineus_ Goldman, Proc. Biol. Soc. Washington, 50:133, September 10, 1937. _Type._--Male, adult, skin and skull, No. 262016, U. S. National Museum (Biological Surveys Collection); Beaverdam Creek, near confluence with Virgin River, Littlefield, 1,500 ft., Mohave County, Arizona; October 16, 1936; collected by Luther C. Goldman; original number 67 (after Goldman, type not seen). _Range._--Extreme southwestern Utah, in Beaverdam Wash, Washington County, Utah. _Diagnosis._--Size medium (see measurements). Color: Upper parts Cinnamon Buff, finely mixed with black; sides and flanks Pinkish Buff; underparts Pale Pinkish Buff; front feet, hind feet, and distal part of tail white; nose, cheeks, chin and top of head grayish black. Skull: Robust, with moderately wide zygomatic arches; zygomatic processes of maxillae wide; zygomatic processes of squamosals long; jugals concave laterally, giving the zygomatic arches the appearance of double bowing; nasals long; extension of premaxillae posterior to nasals long; tympanic bullae well inflated ventrally; pterygoid hamulae heavy; interpterygoid space widely V-shaped; molariform teeth large. _Comparisons._--For comparisons of _virgineus_ with _Thomomys bottae planirostris_ and _T. b. birdseyei_ see accounts under those forms. Topotypical specimens of _virgineus_ can be distinguished from those of _Thomomys bottae trumbullensis_ as follows: Size smaller. Color: Lighter throughout. Skull: Zygomatic arches less widely spreading; jugals more bowed medially; zygomatic processes of squamosals longer; extension of premaxillae posterior to nasals greater; tympanic bullae larger and more inflated ventrally; molariform teeth larger.