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.
Among named races of _T. bottae_, _contractus_ is closest morphologically to _tivius_. It differs from it as follows: Size larger throughout. Color: Lighter throughout. Skull: The same general shape and proportions, but larger in every measurement taken; rostrum longer and narrower; extension of premaxillae posterior to nasals longer; posterior tongues of premaxillae narrower.
_Remarks._--Fifteen animals from Oak City are intergrades between _contractus_ and _tivius_. Intergradation with _lenis_ is also shown in some specimens by the widely spreading zygomatic arches. In the majority of characters including the diagnostic long, slender, narrow rostrum they are more like _contractus_ to which they are here referred.
Nine animals from Beaver were considered by Hall (1931:3) and Durrant (1937:4) to be intergrades between _Thomomys bottae albicaudatus_ and _Thomomys bottae centralis_. Restudy of these specimens in the light of additional material now shows them to be intergrades between _T. b. centralis_, _T. b. planirostris_ and _T. b. contractus_. The majority of these animals are intermediate in color between _centralis_ and _contractus_, but a few have the reddish cast of _planirostris_. The shape of the nasals is characteristic of _planirostris_, while the zygomatic arches are as in _centralis_. In the remainder of the diagnostic characters they are like _contractus_ to which they are here referred.
Strong affinities exist between _albicaudatus_, _tivius_ and _contractus_. All three of these races probably stemmed from a dark form which formerly inhabited the eastern mainland of the Pleistocene Lake Bonneville. At present, _tivius_ is isolated on the Cañon Mountains in eastern Millard County, while the range of _albicaudatus_ and _contractus_ have been separated by that of _lenis_. _T. b. lenis_ has the majority of its affinities with _aureiventris_ which is an inhabitant of the western mainland of this ancient lake. An understanding of the history of the Sevier River Valley will probably clarify this distribution of pocket gophers.