Pappogeomys alcorni Russell, 1957. Univ. Kansas Publ. Mus. Nat. Hist., 9(11):359. Type from 4 mi. W Mazamitla, Jalisco.
Pappogeomys bulleri Thomas, 1892. Ann. Mag. Nat. Hist., Ser. 6, vol. 10:196, August. Type from "near Talpa," west slope of Sierra Madre de Mascota, Jalisco.
1895. Cratogeomys Merriam, N. Amer. Fauna, 8:150, January 31.
1895. Platygeomys Merriam, N. Amer. Fauna, 8:162, January 31. Type: Geomys gymnurus Merriam, 1892.
Type.—Geomys merriami Thomas, 1893, from "Southern México," probably in Valley of México.
Chronologic range.—Late Pleistocene, from Wisconsin deposits (San Josecito Cave, Nuevo León, Upper Bercerra, México, and Burnet Cave, New Mexico, local faunas) to the Recent.
Description.—Size medium to large; skull becoming angular and rugose with age, and tending towards platycephaly and dorso-ventral compression; zygomata stout, each bearing platelike expansion at anterolateral angle into which anterior end of jugal becomes morticed; breadth across zygomata great relative to length of skull; rostrum relatively broad; squamosals expanding medially with age eventually growing over lateral parts of parietals, and sometimes also expanding laterally displacing postglenoid notch; sagittal crest well developed in adults of both sexes, but especially high and bladelike in males; lambdoidal crest prominent in all but young animals, having dorsal outline broadly convex posteriorly in most species but strongly sinuous in gymnurus-group; enamel plate on posterior wall of P4 absent; enamel plates present only on anterior walls of M1 and M2; M3 variform in occlusal shape (as described in species account), either subtriangular (gymnurus-group), quadriform or obcordate (castanops-group, with exceptions as noted before); lateral plates of M3 usually present in all species, labial plate approximately as long as lingual plate in gymnurus-group (like that in subgenus Pappogeomys) or distinctly shorter in castanops-group (labial plate scarcely extending beyond border of labial re-entrant fold); one or both lateral plates tending to disappear with wear in castanops-group, with lingual plate usually disappearing first; breadth across angular processes clearly more than breadth across zygomatic processes, especially in gymnurus-group.
Remarks.—In the species of the castanops-group the skulls can be spoken of as generalized and the least platycephalic of the subgenus. Indeed, the species of the castanops-group are hardly more specialized in this respect than is the subgenus Pappogeomys. In these skulls the breadth across the squamosal processes is less than that across the zygomatic arches, although the two dimensions are almost equal in some examples of P. merriami of the castanops-group (where squamosal breadth varies from 85 to 98% of zygomatic breadth). In the species having marked platycephalic skulls (gymnurus species-group) the breadth across the squamosal processes equals or exceeds the breadth across the zygomatic arches (squamosal breadth rarely 97 to 99% of zygomatic breadth), except in P. zinseri and P. tylorhinus zodius.
The variable character of the third upper molar as between species suggests that this tooth is presently undergoing active evolution. The structure of this tooth, although differing between taxa, is remarkably stable in other kinds of Geomyini. The most remarkable modification of M3 in Cratogeomys is the obcordate pattern developed in P. merriami of the castanops-group. The posterior loph and entire tooth is shortened somewhat resembling in shape that of Thomomys. Moreover, the posterior loph is twisted labially; consequently, its posterior surface now forms the labial border of the weakly defined posterior loph. Owing to the torsion, the lingual enamel plate has been rotated to the posterior surface of the tooth. Therefore, the tooth is provided with two transverse enamel plates, including the plate on the anterior wall of the tooth. The labial plate is greatly reduced, its total surface being restricted to the small labial inflection. The highly specialized obcordate M3 is not found in the most specialized platycephalic skulls characteristic of the gymnurus species-group. Instead the gymnurus-group retains the primitive subtriangular pattern without significant modification.