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.
Specimens examined.—Total, 39, distributed as follows: Millard County: Oak City, 5,000 ft., 15; Scipio, 5,315 ft., 15. Beaver County: Beaver, 6,000 ft., 9 (M. V. Z.).
Thomomys bottae lenis Goldman
Thomomys townsendii lenis Goldman, Proc. Biol. Soc. Washington, 55:75, June 25, 1942.
Thomomys perpallidus aureus Moore, Journ. Mamm., 10:259; November 11, 1931.