Description and discussion.—The size ranges from as little as in the smaller kinds of Thomomys to the maximum attained in the subfamily and matched elsewhere perhaps in only a few of the larger subspecies of Orthogeomys grandis. Depending on the species and subgenus, the form of the skull varies from generalized to specialized. The generalized skulls are short and not especially narrow; the zygomatic arches are spread laterally so far that the breadth across them exceeds the breadth across the mastoid processes. The most specialized skulls are platycephalic and the breadth across the mastoid processes equals or exceeds the breadth across the zygomatic arches (even so, the zygomatic arches are still relatively widespread). In correlation with the great breadth of the posterior part of the cranium, the rami of the mandibles diverge widely posteriolaterally and the angular processes are remarkably elongated. The rostrum is moderately broad in most species, but not nearly so broad and heavy as in Orthogeomys.
The single deep, median sulcus on the outer surface of the upper incisor is slightly displaced to the inner side of the tooth. The posterior surface of P4 lacks enamel (small vestige found on lingual end of posterior wall in only two adult individuals—UA 3260 and KU 100442, of the subgenus Pappogeomys); the other three plates are fully developed as usual. The p4 is provided with four fully developed enamel plates, in the pattern characteristic of the tribe Geomyini. In the p4 of the late Pliocene species (P. bensoni) the re-entrant angles are open (obtuse), a trait that is evidently primitive in the Geomyini.
All three lower molars are single, compressed, elliptical columns with enamel on only the posterior surfaces. M1 and M2 are also elliptical in cross-section and decidedly anteroposteriorly compressed, like the lower molars. Nevertheless, the enamel pattern is variable; enamel plates may be retained completely across both the anterior and posterior walls of M1 and M2 or only the anterior plate may be retained without reduction and the posterior plate may be reduced so that only a vestige is retained on the lingual fourth of the tooth or the posterior plate may be completely lost.
M3 tends to remain at least incompletely bilophodont by reason of retaining a permanent labial re-entrant fold in most species (with exceptions in Pappogeomys bulleri and some old adults of P. castanops). Primitively the occlusal surface of M3 is subtriangular (subgenus Pappogeomys), but in the castanops species-group of the advanced subgenus Cratogeomys, the posterior loph usually is reduced and the occlusal surface is quadriform or obcordate. Curiously, the trend towards reduction of the posterior loph is reversed in one subspecies (P. merriami fulvescens) and, the loph has elongated into a pronounced heel in some specimens, resembling the condition in Orthogeomys. The entire range of variation occurs in P. m. fulvescens. The subtriangular pattern is retained in the most specialized species of Cratogeomys where that pattern is associated with extreme platycephaly in the gymnurus species-group. In most species the posterior loph supports two lateral plates, the outer one always bordering the labial re-entrant fold. In Pappogeomys bulleri and in the castanops species-group, the outer re-entrant fold of M3 tends to be obsolete, and the tooth becomes quadriform or suborbiculate in some individuals and loses the bilophodont pattern that characterizes other species. The lingual enamel plate is displaced to the posterior surface of the tooth, and one or both plates may disappear with advancing age. Consequently, only the anterior enamel plate remains in some adults, and constitutes the maximum degree of reduction of enamel on M3 in the Geomyinae. In many adults of Pappogeomys bulleri, the enamel investment of the posterior loph is complete and the two lateral plates are connected, without interruption around the posterior apex of the tooth, evidently representing the retention of a primitive character of the ancestral lineage.
The m3 of P. bensoni from the late Pliocene is distinguished by minute lateral inflections suggesting the primitive biprismatic pattern. Also the posterior enamel plates of m1 and m2 are remarkably long, extending around the ends of the tooth. The associated upper incisor was unisulcate as in the modern species, and the basitemporal fossa of the mandible is well developed and deep.
The lower jaw is stout and relatively short. The masseteric ridge is well developed and has an especially thick crest. The basitemporal fossa is deep. In most living species, the pelage is soft and dense, but in one species, Pappogeomys fumosus, the hairs are coarse and hispid somewhat as in Orthogeomys.
Key to the Subgenera of Pappogeomys
A Enamel plates completely developed across posterior walls of M1 and M2, except in one species (P. alcorni) having enamel restricted to lingual fourth in M1; sagittal crest lacking owing to impressions of temporal muscles remaining separated (even in old adults); zygomata slender, and without platelike expansion at lateral angle. Subgenus Pappogeomys
A´ Enamel lacking on posterior walls of M1 and M2; pronounced sagittal crest developed in adults of both sexes by union of temporal impressions at middorsal line; zygomata stout and wide, with lateral angle expanded into broad plate. Subgenus Cratogeomys