1. _Thomomys_ 2. _Geomys_ 3. _Zygogeomys_ 4. _Pappogeomys_ 5. _Orthogeomys_ ] _Thomomys_ The earliest Pleistocene records of _Thomomys_ are mostly isolated teeth. Although they can be identified as genus _Thomomys_, most of the materials are too fragmentary to be identified to species. In _Thomomys_ two distinct patterns of occlusal surfaces of the molars can be recognized: the generalized elliptical pattern in the subgenus _Pleisothomomys_, not unlike the pattern in other geomyids, and the pear-shaped pattern in the subgenus _Thomomys_, which results from constriction of the upper molars on the labial side and constriction of the lower molars on the lingual side. Some fossils assigned to _Thomomys_ were not examined with this distinction in mind by the persons who made the assignments. Consequently some of the identifications now in the literature may be subject to change. Three occurrences of _Thomomys_ are from the early and middle Pleistocene, with a possible fourth (depending upon the age of the Hay Springs local fauna of Nebraska). The earliest Pleistocene record is from the Broadwater-Lisco beds along the North Platte River in Morrill County, western Nebraska. Possibly the specimen from there was misidentified. Those beds are Lower Pleistocene, and are regarded by Schultz and Stout (1948:560-561, 573) and by Hibbard (1958:11), as having been deposited mostly during the Aftonian interglacial. There is also some indication that some of the strata were deposited late in the Nebraskan glaciation. There are no other early Pleistocene records of _Thomomys_. Savage (1951:228) reported the genus from the Irvington local fauna, Alameda County, California. The specimens were not identified to species, although they were described as indistinguishable from _Thomomys bottae_. Paulson (1961:137) recorded specimens from the Cudahy local fauna, Meade County, Kansas. These fragmentary specimens are referable to the subgenus _Thomomys_, owing to the strong constriction of the molars, but have not been identified to species. The Cudahy is an Irvingtonian local fauna, and is considered to have been deposited during the late Kansan glaciation. The stratum containing the Cudahy local fauna immediately underlies the Pearlette Ash. The Cudahy material includes five isolated molars and a fragmentary ramus bearing only the premolar. The genus _Thomomys_ has been recovered also from the Hay Springs local fauna in Sheridan County, northwestern Nebraska, by Shultz and Tanner (1957:71). The Hay Springs local fauna is considered to have been deposited in late Kansan glaciation or in early Yarmouth interglacial by Shultz and Tanner (_op. cit._:69), or of Irvingtonian age; however, Hibbard (1958:25) regarded the beds containing this fauna as Illinoian (thus post-Irvingtonian in age), and equivalent in age to the Berends local fauna of Oklahoma and the Butler Springs and Mt. Scott local faunas of Kansas.