Numerous specimens of geomyids from many localities and horizons are available from the Pleistocene of North America. Specimens of the genera _Geomys_ and _Thomomys_ are especially common. Few specimens are known of the genera _Orthogeomys_ and _Pappogeomys_, especially from the early and middle Pleistocene, owing, probably, to slight knowledge of the early Pleistocene of México where these two genera are thought to have evolved (see map, Figure 2). This lack of knowledge about early Pleistocene deposits in México is a handicap in the present instance since the center of differentiation for several of the modern genera is judged to have been in México, probably on, and at the edge of, the Central Plateau. The relative abundance of the remains of _Geomys_ and _Thomomys_ from Pleistocene deposits farther north, and the marked absence of other genera, may mean that _Orthogeomys_ and _Pappogeomys_ did not range northward from southern and central México in most of the Pleistocene. One species of _Pappogeomys_ eventually ranged into the southwestern United States in the late Pleistocene (toward the end of the Wisconsin) and it occurs there today, but the genus is essentially Mexican. The fossil record of _Zygogeomys_, as the genus is here understood, evidently continued in the United States will into the Middle Pleistocene, depending upon the stratigraphic interpretation of the age of the Curtis Ranch local fauna from southeastern Arizona. Hibbard (1958:25) regarded the Curtis Ranch local fauna as Irvingtonian in age, a local fauna that lived either in the late Kansan glacial or the Yarmouthian interglacial, and his correlation is tentatively followed here. In deposits laid down later than those of Irvingtonian age no remains of _Zygogeomys_ have been found. Today a single species exists as a relic in the mountains of central México and _Zygogeomys_ may have retreated southward to its present refugium in the late Pleistocene. Perhaps, _Zygogeomys_ occurred in northern México and the southwestern United States in the early and middle Pleistocene (see Fig. 2), occupying the area between the ranges of _Pappogeomys_ to the south and _Geomys_ to the north. Competition with _Pappogeomys_, and especially _Geomys_, during Irvingtonian time may have extirpated _Zygogeomys_ over most of this area, and by late Pleistocene (Sangamon) much of the former range of _Zygogeomys_ came to be occupied by one or the other of its competitors. The occurrence of _Geomys garbanii_ in southern California (see White and Downs, 1961) and the unidentified species of _Geomys_ in Aguascalientes (Mooser, 1959; for faunal correlation, see Hibbard and Mooser, 1963), both from deposits of Irvingtonian age, supports this suggestion. [Illustration: FIG. 2. Probable distribution of the Subfamily Geomyinae in the early Pleistocene (late Blancan), depicting major areas of differentiation of the modern genera.