_Comparisons._--Compared with topotypes of _Thomomys bottae albicaudatus_, _contractus_ differs as follows: Tail longer. Color: Lighter throughout. Skull: Slenderer, less ridged and angular; rostrum narrower; zygomatic and mastoidal breadths less; ascending processes of premaxillae narrower; posterior tongues of premaxillae narrower; posterior ends of nasals less truncate; zygomatic arches weaker, less angular, and less widely spreading posteriorly; interparietal larger; paroccipital processes weaker; interpterygoid space not as widely V-shaped; upper incisors longer and narrower; molariform teeth smaller. Topotypes of _contractus_ can be distinguished from those of _Thomomys bottae convexus_ by the following: Size larger, tail longer; hind foot larger. Color: Darker throughout. Skull: Longer, narrower, and not as massive; top of skull moderately, as opposed to strongly, convex; nasals arched rather than straight; zygomatic arches neither as widely spreading, angular nor massive; space enclosed within zygomatic arches longer; interparietal larger; interpterygoid space more narrowly V-shaped; upper incisors longer and narrower; molariform teeth much lighter. Comparisons of topotypes of _contractus_ with near topotypes of _Thomomys bottae centralis_ show them to be approximately the same size, but to differ as follows: Color: Darker throughout. Skull: Shorter and slenderer; rostrum narrower; region between posterior tongues of premaxillae narrower and more convex transversally; nasals more truncate; zygomatic breadth less, but arches relatively more widely spreading posteriorly; interparietal larger; interpterygoid space generally narrower; upper incisors longer and narrower; molariform teeth smaller. Topotypes of _contractus_ differ from those of _Thomomys bottae aureiventris_ as follows: Size smaller; tail longer; hind foot shorter. Color: Darker throughout. Skull: Shorter but slenderer; rostrum narrower; nasals shorter but slenderer, and more truncate posteriorly; extension of premaxillae posterior to nasals longer; zygomatic arches weaker and less angular; zygomatic processes of maxillae weaker and with no marked thickenings at union of maxilla and jugals; interparietal larger; interpterygoid space more generally V-shaped; upper incisors longer and narrower; molariform teeth smaller. Compared with topotypes of _Thomomys bottae planirostris_, _contractus_ differs in: Size smaller throughout. Color: Darker, more black and less Cinnamon in pelage. Skull: Smaller in every measurement taken; rostrum narrower; nasals arched instead of flat; zygomatic arches neither angular, massive nor widely spreading; upper incisors narrower; molariform teeth markedly smaller and weaker. Topotypes of _contractus_ differ from those of _Thomomys bottae levidensis_ in larger size, darker color and longer, slenderer skulls.