1933. _Pleisothomomys_ Gidley and Gazin, Jour. Mamm. 14:354. Type: _Pleisothomomys potomacensis_ Gidley and Gazin, from Pleistocene, Cumberland Cave local fauna, Allegany County, Maryland. _Chronologic range._--Known from late Pliocene to Recent. _Description._--Same as that given for the tribe Thomomyini above. _Discussion._--Features characterizing _Thomomys_ and the tribe Thomomyini are more advanced than those characterizing the tribe Dikkomyini. Also, the Thomomyini retain more of the primitive features of the Geomyinae than do the more specialized tribe Geomyini. Specializations are few, but include the third molar being a single column both above and below, enamel plates, and a masseteric ridge. Key to the Subgenera of _Thomomys_ A Molars sub-crescent or ovate in cross-section, not becoming abruptly narrower at one end of tooth. Subgenus _Pleisothomomys_ p. 519 A´ Molars pear-shaped, not sub-crescent or ovate, in cross-section, crown becoming abruptly narrow at one end of tooth. Subgenus _Thomomys_ p. 520 Subgenus =Pleisothomomys= Gidley and Gazin 1933. _Pleisothomomys_ Gidley and Gazin, Jour. Mamm., 14:354, November 13. _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):