The genus _Pappogeomys_, as it is conceived of in this study, is comprised of two subgenera; one, _Pappogeomys_, is generalized and primitive, and the other, _Cratogeomys_, is specialized, and includes the most highly specialized of the modern pocket gophers. The subgenus _Pappogeomys_ is regarded as the ancestral lineage, and the subgenus _Cratogeomys_ is regarded as an early offshoot, probably in the early Pleistocene, that became progressively more specialized in the course of its subsequent evolution. In the same period of time, the subgenus _Pappogeomys_ changed little. It is known only from late Pliocene fragments and from the living species. The ancestral morphotype is preserved in _Pappogeomys_. Primitive characters are: (1) Small size; (2) skull generalized and smoothly rounded; (3) temporal ridges separate (not uniting into a sagittal crest); (4) enamel plates retained on both anterior and posterior walls of M1 and M2; (5) M3 bilophate, its posterior loph short. Basic specializations are few and include loss of the inner groove from the anterior face of the upper incisor; anteroposterior compression of the lateral re-entrant folds of the premolars; and loss of enamel from the posterior wall of P4. All three features have been perpetuated in the advanced subgenus _Cratogeomys_, suggesting that they were already developed in the early evolution of the subgenus _Pappogeomys_ before _Cratogeomys_ diverged. Agreement with _Geomys_ is demonstrated by the lack of enamel on the posterior wall of P4 (see Fig. 9) and by retention of the posterior enamel plate on M1 and M2. In _Pappogeomys (Pappogeomys) alcorni_ the enamel from the posterior face of M1 has been lost from all but the lingual fourth or so of the posterior wall (Fig. 9E). Reduction of enamel in M1 provides an example of parallelism with the more advanced subgenus _Cratogeomys_, discussed below.