_Comparisons._--From topotypes of _Thomomys bottae aureiventris_, _bonnevillei_ differs as follows: Size smaller, hind foot shorter. Color: Upper parts and sides lighter; underparts pale buff rather than "gold." Skull: Shorter and relatively wider; rostrum wider and heavier; zygomatic arches relatively wider and more massive, with greatest width posteriorly instead of anteriorly; interpterygoid space widely V-shaped rather than lyre-shaped; thickening at union of jugal and zygomatic process of maxilla less developed; anterior palatine foramina larger; nasals shorter and more markedly flared distally; zygomatic breadth relatively, and mastoidal breadth actually, wider; extension of premaxillae posterior to nasals greater; tympanic bullae more inflated ventrally; upper incisors wider. From near topotypes of _Thomomys bottae centralis_, from 1 mile east of Garrison, Millard County, Utah, _bonnevillei_ differs as follows: Size smaller; hind foot and tail shorter. Color: Generally darker above and lighter below; top of head darker; postauricular patches smaller and lighter. Skull: Shorter and wider (zygomatic breadth expressed in percent of basilar length being, in males, 74.5 in _bonnevillei_ and 71.5 in _centralis_); interpterygoid space more widely V-shaped; interparietal smaller, and more triangular; nasals shorter and much more dilated distally, as well as more constricted proximally; lacrimal processes smaller and less globuse at tips; temporal fossae larger; braincase and entire dorsal surface of skull more nearly flat; lambdoidal suture convex posteriorly as opposed to nearly straight; tympanic bullae more inflated ventrally. Comparisons of _bonnevillei_ with the type and type series of _Thomomys bottae wahwahensis_ show them to be of approximately the same size, but to differ as follows: Color: Slightly darker above and lighter below; postauricular patches smaller and lighter. Skull: Larger in every measurement taken, except breadth of rostrum which is smaller; skull not as flat; tympanic bullae more inflated ventrally; nasals and rostrum longer; extension of premaxillae posterior to nasals greater; interparietal smaller and more triangular; zygomatic arches more bowed out laterally; jugals heavier; interpterygoid space more widely V-shaped; upper incisors less massive. The characters that distinguish _bonnevillei_ from _Thomomys bottae albicaudatus_ are: Size smaller. Color: Markedly lighter throughout. Skull: Shorter and wider; mastoid and zygomatic breadths greater; rostrum narrower but shorter; angle between rostrum and zygomatic processes of maxillae less; interparietal smaller and more triangular; extension of premaxillae posterior to nasals greater; upper incisors shorter, narrower and more recurved.