Type.—Pleisothomomys potomacensis Gidley and Gazin, 1933.
Chronologic range.—Late Pliocene (Hagerman local fauna, Idaho) to late Pleistocene. The latest records are from the fauna of Saber-tooth Cave, Florida, a late Pleistocene assemblage that probably was deposited in the Sangamon. The middle and late Pleistocene records are from the eastern United States, suggesting that the subgenus Pleisothomomys was restricted to that region while the subgenus Thomomys occupied the western United States and parts of Canada and México as it does today.
Description and Comparison.—Separated from subgenus Thomomys only on basis of sub-crescentic shaped molars (only jaw fragments and isolated teeth known), seemingly a primitive feature of the genus. This dental structure continued into the late Pleistocene; none of the Recent species expresses this feature of the molars, although the molars of Thomomys vetus of the late Pleistocene (Wisconsin deposits), referred to the subgenus Thomomys on the basis of its alleged relationship to Thomomys townsendii (see Davis, 1937:156-158), are less distinctly pear-shaped, and are more sub-crescentic, than in any other known species of the subgenus Thomomys. Pleisothomomys Gidley and Gazin (loc. cit.) was proposed as a genus but is here considered as of no more than subgeneric worth, and is recognized because of the apparent constancy of the sub-crescentic molars in the earlier members of the genus and in those populations of Thomomys occurring in Pleistocene times in the eastern United States.
Referred species.—Three (all extinct):
*Thomomys gidleyi Wilson, 1933. Carnegie Inst. Washington Publ. 440:122, December. Type from Hagerman beds, late Pliocene, Idaho.
*Thomomys potomacensis Gidley and Gazin, 1933. Jour. Mamm., 14:354, November 13. Type from Cumberland Cave, middle and late Pleistocene, Maryland.
*Thomomys orientalis Simpson, 1928. Amer. Mus. Novit., 328:6, October 26. Type from Saber-tooth Cave, late Pleistocene, Florida.
Subgenus Thomomys Wied-Neuwied
1839. Thomomys Wied-Neuwied, Nova Acta Phys.-Med. Acad. Caesar. Leop. Carol., 19(1):377.