which Franzen (1947:58) wrote. She assigned them to the genus Geomys, and they may actually be a primitive form of Geomys that represents an intermediate stage in the development of the enamel pattern from the uninterrupted loops of the ancestor to the discontinuous pattern of modern Geomys. I favor this interpretation; the evidence, however, is inconclusive, and I have, therefore, reluctantly allocated them, along with the other specimens of Nerterogeomys, to the genus Zygogeomys. In an early paper, Hibbard (1938:244) erroneously referred the same specimens, two upper premolars of a young individual, to the genus Thomomys, and the same material was identified with the genus Geomys, also without specific assignment, in a later paper (Hibbard, 1941b:278). Thomomys is unknown from the late Pliocene of the Great Plains. The specimens previously referred to Nerterogeomys are assigned to the genus Zygogeomys for the first time in this report; for a discussion of the systematic arrangement see the accounts beyond. The type and paratype of Nerterogeomys from the Benson local fauna of Arizona have no indication of enamel reduction.

Specimens of the genus Geomys from the late Pliocene were referred to the large Geomys quinni McGrew, first by Franzen (1947:55) and later by Hibbard and Riggs (1949:835) and Hibbard (1950:171). Geomys quinni has been obtained from the Fox Canyon locality and Locality 3 of the Rexroad local fauna. At Locality 3, both Zygogeomys (cf. minor) and Geomys quinni have been found together, but Geomys quinni can be distinguished by its much larger size and the advanced enamel pattern of the cheek teeth (see systematic accounts beyond). All age classes are represented among the specimens of Geomys quinni; therefore, it seems unlikely that the smaller gophers referred to Zygogeomys are actually the young of Geomys quinni. Hibbard (personal communication, May, 1966) informed me that specimens of Geomys from the late Pliocene (Fox Canyon and Rexroad Locality 3) are erroneously referred to G. quinni. According to Hibbard, this material represents instead two distinct undescribed species, descriptions of which have been submitted by him for publication. Allocation of late Pliocene specimens of Geomys quinni to other species will restrict quinni to the early Pleistocene.

Cratogeomys bensoni Gidley (1922:123) was of medium size. The name was based on an upper incisor bearing a single median sulcus and an associated lower jaw containing all of the cheek teeth from the Benson local fauna, Cochise County, Arizona. Additional lower jaws carrying various teeth also were recovered. The specimens might just as well have been assigned to the genus Pappogeomys since the lower dentitions of all the genera of the tribe Geomyini have the same enamel pattern, and the subgenera Pappogeomys and Cratogeomys have upper incisors with median grooves. The specimens are too fragmentary to warrant more than generic identification. Mainly because of their late Pliocene age and primitive traits the specimens are here regarded as early representatives of the subgenus Pappogeomys. Discovery of the upper molariform dentition would make a more precise assignment possible.

Pleistocene

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 p. 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.