supposed early appearance of Geomys and Thomomys led to much confusion concerning geomyid evolution in the late Tertiary.

The dearth of geomyines in the Miocene is counterbalanced by the relatively abundant and highly differentiated gophers of the subfamily Entoptychinae. They reached the zenith of their development in this period. Four genera and a number of species are known from the western part of the United States, mostly from beds along the Pacific Coast and in the northern part of the Great Plains. The great diversification of the group in a relatively short period suggests prior movement into a new adaptive zone and subsequent specialization in different subzones and therefore an episode of radial adaptation. The radiation of the entoptychines is discussed elsewhere in the account of geomyid phylogeny, but it should be noted here that both the Geomyinae and the Entoptychinae appear in the fossil record at about the same time in the early Miocene. The principal distinguishing features of each of the two lineages were well developed at the time of their first occurrence, and the entoptychines were the more successful in early Miocene. The Entoptychinae are known only from the early and middle Miocene, unless the earlier deposits of the John Day Formation of Oregon from which mammals have been recovered are considered to be latest Whitneyian (latest Oligocene); for correlations, see Wilson (1949:75). Both lineages likely had an earlier history extending back to their divergence in the Oligocene.

Pliocene

The oldest and most primitive Pliocene geomyine is Pliosaccomys dubius Wilson (1936:20) from the Smith Valley local fauna of middle Pliocene (Hemphillian) age in Nevada. According to Wilson (op. cit.:15) the beds probably were deposited near the middle of Hemphillian time. Shotwell (1956:730) recorded Pliosaccomys dubius from the McKay Reservoir and from the Otis Basin (1963:73) local faunas of the middle Pliocene (Hemphillian) of Oregon, and Green (1956:155) has recovered remains of Pliosaccomys (cf. dubius) from the Wolf Creek local fauna, uppermost part of the lower Pliocene (late Clarendonian in age), of Shannon County, South Dakota. Recently, James (1963:101) has described a second species, Pliosaccomys wilsoni, of this primitive genus. The new species was found in early Pliocene deposits (late Clarendonian) from the Nettle Spring local fauna (Apache Canyon), in the Cuyama Valley, Ventura County, California. Pliosaccomys wilsoni does not differ greatly from P. dubius; however, the few differences in dental characters seem to warrant specific recognition. The reduction of cusps on the metalophid of p4 from three (dubius) to two (wilsoni) and the lack of accessory cuspules on the protolophid of p4 in wilsoni are probably specializations, suggesting that P. dubius even though the more recent in age is the less advanced of the two. P. wilsoni is known only from a lower jaw of a young individual that had dp4 in place, along with m1 and m2. The permanent premolar was in the process of erupting, and the deciduous tooth was removed so that the unworn surface of p4 could be examined.

Pliosaccomys occurred geographically in the area that the Entoptychinae had occupied in the early Miocene. The Smith Valley material includes dentitions in almost all stages of wear and the chronological sequences in the development of the patterns of wear can be reconstructed. An understanding of the dental patterns of the primitive geomyines is based mostly on the interpretation of the stages of wear in Pliosaccomys.

No other pocket gopher is known from the area in which Pliosaccomys occurred, and it is unknown after middle Hemphillian age. Pliosaccomys has closer affinities with Dikkomys of the early Miocene than with any geomyid of the modern assemblage and gives no clue to the origin of the lineage culminating in the modern pocket gophers of the tribe Geomyini.

Pliogeomys buisi Hibbard (1954:353) was found in the Buis Ranch local fauna, of latest middle Pliocene, on the west side of Buckshot Arroyo, Beaver County, Oklahoma. The original material included a right ramus bearing the premolar and first two molars (the holotype) and five isolated premolars and molars. One of the molars is slightly worn and from an immature individual. One premolar is a deciduous tooth. Hibbard (op. cit.:342) identified the beds from which he obtained the Buis Ranch local fauna as from the lowermost part of the Upper Pliocene. Moreover, he judged the Buis Ranch local fauna to be only slightly older than the Saw Rock Canyon local fauna of Seward County in southwestern Kansas. Previously (Hibbard, 1953:408-410), the Saw Rock Canyon local fauna had been assessed as older than the Rexroad local faunas (latest late Pliocene) and, therefore, representative of the early part of the late Pliocene. More recently, Hibbard (1956:164) identified the Buis Ranch beds as part of the Ogallala Formation, which here occurs unconformably just beneath the Rexroad Formation (composed of strata nearly all of late Pliocene age). Therefore, he regarded the Buis Ranch beds as latest middle Pliocene in age. Hibbard (1954:356) suggested that pocket gopher remains from the Saw Rock Canyon local fauna were referable to Pliogeomys buisi, and, in effect, tentatively assigned them to Pliogeomys (in his description of the genus Hibbard remarked that the upper incisor is bisulcate as in Geomys, and the only upper incisor that he mentions was one of the Saw Rock Canyon fossils and not part of the Buis Ranch material). Pliogeomys has closer affinities with modern pocket gophers of the tribe Geomyini than it does with the middle Pliocene genus Pliosaccomys.

The pocket gopher fauna known from the late Pliocene was more varied than the faunas known from any earlier time. In addition to the extinct Pliogeomys, which occurs in early late Pliocene (see discussion above), the living genera Zygogeomys, Geomys, Pappogeomys (in the sense used on p. 534), and Thomomys first appear in the late Pliocene. The only other living genus, Orthogeomys, makes its first appearance in the late Pleistocene.

The earliest record of the genus Thomomys is based on a fragment of a left mandibular ramus bearing p4 and m1, Thomomys gidleyi Wilson (1933b:122), from the Hagerman local fauna of Twin Falls County, Idaho. Wilson (loc. cit.) was uncertain as to age (late Pliocene or early Pleistocene) but subsequently (1937:38 and 67-70) settled on the middle part of the late Pliocene. Hibbard (1958:11) later considered the age as early Pleistocene (suggesting that the deposits accumulated in the Aftonian interglacial interval) but subsequently (Hibbard et al., 1965:512), on the basis of potassium argon age determinations, also settled on late Pliocene.

Remains of Nerterogeomys [=Zygogeomys] have been found in the Benson local fauna, Cochise County, Arizona, and the Rexroad local fauna of Kansas. This early Blancan gopher first was described as Geomys minor by Gidley (1922:123), and was later referred by Gazin (1942:487) to his new genus Nerterogeomys. Hibbard (1950:138) identified specimens from the Fox Canyon locality, one of the localities of Meade County, Kansas, where the Rexroad local fauna is preserved, as Nerterogeomys, and tentatively referred them to the species N. minor. Nerterogeomys cf. minor has been recovered also from Locality 3 of the Rexroad local fauna (Hibbard, 1950:171) of Meade County, Kansas. Apparently these are also the small gophers about