_Thomomys bottae_ has been identified from Wisconsin age deposits in western North America, as follows: Burnet Cave, Gaudalupe Mt., New Mexico (Schultz and Howard, 1935:280); Carpinteria Asphalt, California (Wilson, 1933a:70); McKittrick Asphalt, Kern County, California (J. R. Schultz, 1938:206); Rancho La Brea, Los Angeles County, California (Dice, 1925:125--specimens described as a new subspecies, _T. b. occipitalis_); Papago Springs Cave, Santa Cruz County, Arizona (Skinner, 1942:150 and 158--probably _bottae_, but possibly _umbrinus_ on the assumption that the two are specifically instead of subspecifically distinct); Isleta Cave, Bernalillo County, New Mexico (Harris and Findley, 1964:115--some of these fossils may be post-Wisconsin in age); Potter Creek Cave and Samwel Cave, Shasta County, California (Sinclair, 1905:146--identified as _T. leucodon_, now a subspecies of _T. bottae_; also see Hay, 1927:214-215). _Thomomys umbrinus_ has been reported from San Josecito Cave, Nuevo León, México (Russell, 1960:542); Upper Bercerra, México (Hibbard, 1955a:51--identified only as _Thomomys_ sp., but undoubtedly referable to _T. umbrinus_). Post-Wisconsin remains of _Thomomys umbrinus_ are reported by Alvarez (1964:6) from capa II and capa III of the Cueva La Nopalera, southwestern Hidalgo. Hay (1927:222-223) reported specimens of the genus _Thomomys_ from Wisconsin deposits in Hawver Cave, Eldorado County, California, but did not assign them to species. Gilmore (1947:158) found the remains of _Thomomys umbrinus_ in cave deposits near Quatro Ciénegas in central Coahuila. These cave deposits may have been laid down during the Wisconsin, but more likely accumulated in the post-Wisconsin. _Zygogeomys_ Remains found in the Curtis Ranch local fauna, Cochise County, in southeastern Arizona are regarded as of middle Pleistocene age. See Gazin (1942:481-484), Wilson (1937:39-40), Hibbard (1958:25), and Hibbard _et al._ (1965:510-511). Although some question as to the exact age of the Curtis Ranch local fauna still seems to exist, most authorities on the Pleistocene agree that the age is not Pliocene and that it is older than Rancholabrean. Gidley (1922:122) described the pocket gopher found in the Curtis Ranch beds as _Geomys parvidens_, which is preoccupied by _Geomys parvidens_ Brown (1908:194), a name proposed for the pocket gopher from the Conard Fissure of Arkansas; therefore, Hay (1927:136) proposed the name _Geomys persimilis_ for the Curtis Ranch species to replace _Geomys parvidens_ Gidley. _Geomys persimilis_ Hay became the type species of Gazin's genus _Nerterogeomys_ (1942:507). In this paper, _Nerterogeomys_ is considered to be a junior synonym of _Zygogeomys_.