Comparisons of _robustus_ with topotypes of _Thomomys bottae albicaudatus_ show the following: Size smaller. Color: Lighter throughout; postauricular patches smaller and lighter. Skull: Smaller, more compact and more nearly flat; rostrum shorter and more nearly straight; lacrimal processes larger, projecting higher above the anteromedial angle of the orbit; parietal ridges uniformly heavier; mastoid width actually as well as relatively wider; zygomatic arches heavier and relatively much wider (males 76.2 percent of basilar length, females 73.8 percent as opposed to males 73.8 percent and females 73.5 percent); union of jugal and zygomatic process of maxilla uniformly more thickened; spinous process at jugal-maxillary suture present; zygomatic arches much more concave on ventral surface; uniform deep depression present in mature adults, between frontal processes of premaxillae, and anterior interorbital region of frontals; extension of premaxillae posterior to nasals less; sphenorbital fissure more constricted; tympanic bullae more inflated ventrally, extending well ventrad of basioccipital; palatal pits shallower and smaller; molars smaller; upper incisors shorter, narrower and paler (see comparison of _aureiventris_). From near topotypes of _Thomomys bottae centralis_ from 1 mile east of Garrison, Millard County, Utah, _robustus_ differs in: Size smaller; tail and hind foot shorter. Color: Lighter, terminal bands of hair cinnamon, but because more black in underfur the animals appear darker; postauricular patches smaller and lighter. Skull: Shorter, more nearly flat and much more heavily ridged; nasals shorter; rostrum shorter and wider; lacrimal processes larger and projecting higher above anteromedial angle of orbit; zygomatic arches heavier, shorter, more angular and actually as well as relatively wider; jugals thicker; angle between maxillary plate and rostrum less obtuse; spinous process at jugal-maxillary suture present; extension of premaxillae posterior to nasals less; parietal ridges much more pronounced; looked at from above, space enclosed within zygomatic arches more quadrangular in shape as opposed to roughly triangular; tympanic bullae more inflated ventrally; molars smaller; upper incisors shorter, narrower and paler. The characters that distinguish _robustus_ from topotypes of _Thomomys bottae wahwahensis_ are: Size slightly smaller. Color: Darker throughout. Skull: Rostrum longer and narrower; nasals longer; zygomatic arches wider and longer; lacrimal processes larger and projecting higher above anteromedial angle of the orbit; parietal ridges more roughened; tympanic bullae much larger and more inflated ventrally; supraoccipital higher; middorsal depression in frontals present. For comparisons with _Thomomys bottae bonnevillei_ see account of that form.