A lower molar of Geomys is a single elliptical column, and enamel is restricted to the posterior surface as in Zygogeomys, Orthogeomys, and Pappogeomys. Paulson (loc. cit.) found a thin enamel plate on the anterior surfaces of the lower molars in about five per cent of the individuals of Geomys tobinensis from the Cudahy local fauna (middle Pleistocene, deposits of the late Kansan glaciation). An anterior plate is unknown in other members of the tribe Geomyini, except in the primitive genus Pliogeomys of the middle Pliocene. Occurrence of the plate in Geomys tobinensis is an atavistic trait. Primitive dental patterns occur occasionally in geomyids, as pointed out above, but the frequency of occurrence in G. tobinensis is higher than would be expected.
M1 and M2, like the lower molars, are elliptical in cross-section. Complete enamel plates on the anterior and posterior surfaces are separated by tracts of dentine on the sides of each tooth. M3 is usually suborbicular (sometimes subtriangular) in cross-section. The tooth is not especially elongated posteriorly and usually has no definite heel; therefore, it is not significantly longer than wide. Living species of Geomys rarely have a well defined outer re-entrant fold on M3; less than 10 per cent of the individuals (and usually only one side in each individual in which it occurs) have it, although a shallow inconspicuous groove occurs more frequently. The biprismatic molar characteristic of the ancestral morphotype is less often found in Geomys than in any other living member of the tribe Geomyini. The outer re-entrant fold and biprismatic pattern are more often present in the extinct species Geomys garbanii of the Middle Pleistocene than in other species. Less than 24 per cent of the third upper molars in Geomys garbanii lack a tract of the re-entrant fold and more than 38 per cent have a well developed outer fold (see White and Downs, 1961:13, 18). The bicolumnar pattern, although incomplete, would be clearly evident in those teeth having a well marked re-entrant fold; the pattern occurs less frequently in those teeth with no fold or only a slight one. M3 of geomyids is not usually recovered and, therefore, the occlusal pattern of M3 is unknown in most extinct kinds of Geomys. In Recent Geomys the fold is more common in the eastern pinetis species-group than in the western bursarius species-group.
The masseteric ridge on the outer side of the mandible is well developed in all species of the genus. The position of the mental foramen relative to the anterior part of the ridge varies with individuals and according to species. The basitemporal fossa is always present, but is shallower in the late Pliocene and Pleistocene species than in Recent species. The angular process is short.
Referred species.—The twelve species, five of which are extinct, are as follows:
quinni species-group
*Geomys quinni McGrew, 1944. Geol. Ser., Field Mus. Nat. Hist., 9 (546):49, January 20. Type from Sand Draw local fauna (late Pliocene), Brown County, Nebraska; also known from Broadwater-Lisco local faunas (early Pleistocene), Morrill and Garden counties, Nebraska, Deer Park local fauna (early Pleistocene), Meade County, Kansas.
*Geomys paenebursarius Strain, 1966. Bull. Texas Memorial Mus., 10:36. Type from Hudspeth local fauna (early Pleistocene), Hudspeth County, Texas.
*Geomys tobinensis Hibbard, 1944. Bull. Geol. Soc. Amer., 55:736. Type from Tobin local fauna (middle Pleistocene), Russell County, Kansas; also known from Cudahy local fauna (middle Pleistocene), Meade County, Kansas.
*Geomys garbanii White and Downs, 1961. Contrib. Sci., Los Angeles Co. Mus., 42:1-34, June 30. Type from Vallecito Creek local fauna (middle Pleistocene), San Diego County, California.
*Geomys bisulcatus Marsh, 1871. Amer. Jour. Sci., 3:121. Type from Loup River fossil beds, near Camp Thomas, Nebraska (probably late Pleistocene).