The _Thomomys_ from Hay Springs local fauna has not been referred to species. The relative abundance of _Geomys_, and rarity of _Thomomys_, in Great Plains fossil beds of early and middle Pleistocene is probably due to allopatric distributions of the two genera. The Great Plains area was evidently the center of distribution and differentiation of _Geomys_. Perhaps _Thomomys_ evolved earlier to the west, in the Great Basin and Pacific Coastal regions, and not on the Great Plains. Upper Pleistocene records of _Thomomys_ are more common. The genus was widespread in beds identified with the Illinoian and Sangamon and extended its range eastward to the Atlantic Coast. Stephens (1960:1961) reported _Thomomys_ from the Doby Springs local fauna, Harper County, northwestern Oklahoma. The material (34 isolated teeth) was too fragmentary to permit assignment to species. The molars are constricted on one side, indicative of the subgenus _Thomomys_, like the Cudahy specimens reported by Paulson (see discussion above). Stephens erroneously mentioned that the enamel plate on the posterior face of the upper premolar is unique in _Thomomys_; this plate occurs also in _Zygogeomys_. The Doby Springs local fauna was recovered from beds that have been identified as Illinoian deposits, and it is correlated with the Berends local fauna in Beaver County, Oklahoma, and the Butler Springs local fauna in Meade County, Kansas (see Stephens, _op. cit._: 1700). Local faunas in Maryland and Florida of Rancholabrean age include _Thomomys_, in every instance referable to the subgenus _Pleisothomomys_ on the basis of unconstricted molars. _Thomomys potomacensis_ (Gidley and Gazin, 1933), from Cumberland Cave local fauna, Allegany County in western Maryland, is the type of the genus _Pleisothomomys_ Gidley and Gazin (1933:354). _Pleisothomomys_ is here regarded as a subgenus. The material used in the original description included four lower jaws, one with a complete dentition. Hibbard (1958:25) pointed out that the Cumberland Cave assemblage is a composite fauna including both glacial and interglacial forms. He placed the stratigraphic position of the fauna as definitely Upper Pleistocene, probably deposited in both Illinoian glaciation and during the Sangamon interglacial. _T. potomacensis_ is significantly larger than _T. orientalis_ Simpson (1928:6), from the Saber-tooth Cave local fauna, Citrus County, Florida. Simpson's material included a rostral fragment with an incisor, premolar, and first molar. The Saber-tooth Cave local fauna is regarded by Kurten (1965:219) as having been recovered from Sangamon deposits. _Thomomys_ is unknown from Wisconsin deposits in the eastern United States, and today the genus does not occur east of the Great Plains. _Thomomys_ of Rancholabrean provincial age from the western United States and México is known only from Wisconsin beds.