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.