The species of _Geomys_, both living and extinct, form a distinct and well-marked group. The genus is less primitive in most respects than _Zygogeomys_ and _Orthogeomys_ and it is less specialized than _Pappogeomys_, excluding the ancestral stock (subgenus _Pappogeomys_). Some specimens of species of Irvingtonian age (_Geomys tobinensis_ and _Geomys garbanii_, especially the former) retain primitive enamel plates as does _Zygogeomys_; but this is true of only a small percentage of the individuals. Also the adult dental pattern developed somewhat later in ontogeny in these middle Pleistocene species of _Geomys_ than in either Recent or late Pliocene and early Pleistocene representatives (_Geomys paenebursarius_, _Geomys quinni_) of the genus. Whether these features represent a stage in the evolution of the late Pleistocene and Recent species or a terminal stage in members of a sterile and primitive branch of the main line of evolution of _Geomys_ is uncertain. At present I favor the latter explanation, and view _G. paenebursarius_ and _G. quinni_ as early progressive species that evolved dental specializations that were maintained in the main line of phylogeny. Hibbard proposed the generic name _Parageomys_ (1944:55), but later regarded it as a subgenus of _Geomys_ (1956:182) that includes those species retaining continuous enamel bands until relatively late in ontogeny; no other differences have been noted. When the early phylogeny of _Geomys_ is better understood, _Parageomys_ may serve as a subgeneric taxon in which the primitive species of _Geomys_ can be grouped, but as of now _Parageomys_ is arranged as a synonym of _Geomys_. _Pappogeomys_ and _Cratogeomys_ also form a natural group. Their close relationship is best reflected in formal taxonomy by including them in the same genus. Their dissimilarities are of the sort that separate a primitive ancestral lineage from a divergent and progressively more specialized assemblage. The fossil record is inadequate, and I can only speculate that _Cratogeomys_ diverged from primitive _Pappogeomys_-stock in the earlier Pleistocene, at least before the end of the Irvingtonian. _Cratogeomys_ probably originated on the Mexican Plateau and probably underwent its subsequent evolution there. The living species of the subgenus _Pappogeomys_ are evidently relics of the ancestral stock of the genus. Hooper (1946:397), I think correctly, considered _Platygeomys_ as congeneric with _Cratogeomys_, although the highest degree of specialization of the genus is attained in those species formerly classed in the genus _Platygeomys_. Even so, in my opinion, the differences are insufficient to warrant even subgeneric recognition. CLASSIFICATION Family GEOMYIDAE Gill, 1872