Isolated teeth, to which the name _Geomys bisulcatus_ probably applies, from Illinoian deposits on the Great Plains, show that the dentition characteristic of the living _Geomys_ had been developed by that time. Actually, the Illinoian material is too fragmentary to show clearly its taxonomic or phyletic affinities with the species of the later Pleistocene. Even so, the two main stocks of living _Geomys_, _G. bursarius_ and _G. pinetis_, had certainly been differentiated by Sangamon time. The other living species evidently evolved from one or the other of these two stocks in a period of isolation from the main population, probably in either the Wisconsin or post-Wisconsin. For example, _Geomys arenarius_ clearly differentiated from populations of _Geomys bursarius_ that were isolated by the eastward retreat of the main population from the southwestern United States as that region became more arid in the post-Wisconsin. In review, it seems that the Recent species, represented basically by _bursarius_ and _pinetis_, evolved from Illinoian species (_Geomys bisulcatus?_), which descended in turn from the more primitive species of the early Pleistocene, possibly _Geomys paenebursarius_ or possibly from descendants of the Saunders species. Actually the Saunders species may prove to be _Geomys paenebursarius_. At any rate, three trends that took place during the Pleistocene stage of evolution, in the direction of the modern species, were an increase in size, progressive loss of the posterior enamel plate on P4, and a decrease in the vertical depth of the enamel cap as a result of which the dentine is reached in the initial phases of attrition on the tooth of a juvenile. _Geomys garbanii_, occurring at the periphery of the range of the genus, is regarded as a sterile offshoot of the primitive _tobinensis_-line of evolution. Orthogeomys