1817. Diplostoma Rafinesque, Amer. Monthly Mag., 2(1):44-45, November. Included species: Diplostoma fusca Rafinesque [= Mus bursarius Shaw] and Diplostoma alba Rafinesque [= Mus bursarius Shaw] from the Missouri River region.
1820. Saccophorus Kuhl, Beitr. Zool. und Vergl. Anat., pp. 65, 66. Type: Mus bursarius Shaw, from upper Mississippi Valley.
1823. Pseudostoma Say, Long's Expd. Rocky Mts., I, pp. 406. Type: Pseudostoma bursaria [= Mus bursarius Shaw], from upper Mississippi Valley.
1825. Ascomys Lichtenstein, Abh. K. Akad. Wiss. Berlin (1822), p. 20., fig. 2. Type: Ascomys canadensis Lichtenstein [= Mus bursarius Say], probably from upper Mississippi Valley.
1944. Parageomys Hibbard, Bull. Geol. Soc. Amer., 55:735, June. Type: Parageomys tobinensis Hibbard, from Pleistocene, Cudahy (Tobin) local fauna, Russell Co., Kansas.
Type.—Geomys pinetis Rafinesque, 1817, restricted to Screven County, Georgia, in region of the pines.
Chronologic range.—Late Pliocene faunas of Blancan age (Rexroad, Kansas, and Sand Draw, Nebraska, local faunas) to Recent. Reported from numerous Pleistocene deposits of all stratigraphic levels, especially from the Great Plains, where common today.
Description and discussion.—Pocket gophers of this genus are medium-sized geomyids; none is so small as the average-sized Thomomys. The skull is generalized and lacks the dolichocephalic and platycephalic specializations seen in the genera Orthogeomys and Pappogeomys, respectively. Geomys closely resembles Zygogeomys, but retains fewer of the primitive characters of the ancestral stock. At the same time, Geomys has several specializations. Even so, a considerable amount of parallelism is evident in the phyletic trends of the two genera.
The upper incisor of Geomys is bisulcate as in Pliogeomys and Zygogeomys; the deeper grove is medial and the shallower grove lies near the inner border of the tooth. The premolar, above and below, is bicolumnar; and two columns are joined at their mid-points (deep re-entrant angles separate the columns at the sides). A permanent enamel plate protects the anterior face of the anterior loph, and enamel bands outline each of the re-entrant folds. In p4 a complete enamel plate covers the posterior surface of the posterior loph. All of the enamel bands are interrupted by tracts of dentine, except in the initial stages of wear of the occlusal surface of the newly erupted tooth. For a short time in living Geomys, the enamel bands are continuous as observed in juveniles of Geomys bursarius major (KU 5628, 8531, and 41540). But, the enamel cap is thin and the dentine tracts, which are high on the sides of the tooth, are soon revealed by a minimum of wear on the crown. Therefore, the adult, or final, pattern characterized by interrupted enamel plates emerges early in life and remains throughout the life of the individual. Evidence from fossil Geomys, especially from specimens from early and late Pleistocene deposits, suggests that the final adult pattern appears later, ontogenetically, than in Recent specimens. Some of the fossil premolars in initial stages of wear have continuous and uninterrupted bands of enamel. Geomys quinni of the late Pliocene and early Pleistocene has the interrupted pattern seen in late Pleistocene and Recent Geomys. Also, in late Pliocene and early Pleistocene species, the re-entrant folds diverge laterally and form "open" angles. In later taxa (middle Pleistocene to Recent) the folds are compressed and parallel-sided, and the "open" folds are found only in the early stages of wear.
The posterior enamel plate of P4 disappears in the final stages of wear as the interrupted enamel pattern is formed. In the late Pleistocene and Recent Geomys, the loss of the posterior plate occurs early in life, usually in the first phases of wear on the occlusal surface of the newly erupted tooth, but in fossils of Geomys of corresponding ontogenetic age from the early and middle Pleistocene, the posterior plate is retained in some individuals until a later phase of wear, thereby delaying the appearance of the final pattern. Indeed, in five or fewer per cent of the individuals (see Paulson, 1961:138-139; and White and Downs, 1961:18) a vestige of enamel is retained throughout life or at least until late in adulthood. In Geomys tobinensis, for example, a thin, but transversely complete, plate of enamel occurs all the way down to the base of the loph (Paulson, loc. cit.) and would persist throughout life. In Geomys garbanii, a vestige on the lingual side of the posterior surface of a fully adult specimen was noted by White and Downs (loc. cit.). Vestiges of the posterior plate occur less frequently in living geomyids. Paulson (loc. cit.) found a posterior plate in one of 75 specimens of Geomys bursarius dutcheri. A young (suture present between exoccipitals and supraoccipital) female of Geomys pinetis austrinus (KU 23358) has a vestige of the posterior plate on the lingual side of the tooth as White and Downs (loc. cit.) observed in a specimen of Geomys garbanii. The enamel, I suspect, tends to be thicker on the lingual than on the labial side of the loph and extends farther down the lingual surface in some individuals; therefore, wear on the occlusal surface erodes it down to the dentine more rapidly on the labial than on the lingual side. The tendency of enamel to be retained is a primitive feature.