Increase in size of body and skull is seen in most Pleistocene lineages of the Geomyini. Judging from the smallness of the skull in late Pliocene species, representing the base of three of these lineages, the ancestral species of the living assemblage were no larger than the living species of the subgenus _Pappogeomys_ or the smaller subspecies of _Geomys bursarius_. The recorded range of variation in condylobasal length is 36.1 to 45.5 in _Pappogeomys bulleri_, including both adult males and females. Probably the skulls of the ancestral species were not significantly larger. Maximum dimensions of males in living species are 74.5 (subgenus _Cratogeomys_) and 75.0 (subgenus _Orthogeomys_). These are more than twice the minima observed in _Pappogeomys bulleri_. Zygogeomys This is the least specialized and most primitive of the four lineages, has a generalized type of skull, two grooves on the anterior face of each upper incisor, an enamel plate on the posterior wall of P4, open or divergent lateral re-entrant angles on the premolars, and a bicolumnar and elongated M3. All of these features are primitive and essentially as in the ancestral morphotype. No other modern genus retains so much of the primitive structure. Phyletic trends in _Zygogeomys_ are not well documented in the fossil record; and only a few fossils are known and they are fragmentary as discussed before. The genus is represented in the late Pliocene (_Z. minor_), middle Pleistocene (_Z. persimilis_), and Recent (_Z. trichopus_). The living species is a relict population in the mountains of Central México. Judging from the known material, the phyletic trends in the genus have been increase in size, reduction of enamel on the posterior face of P4 (occurring only in the living species) where a short enamel plate is retained on the lingual side of the tooth (see Fig. 7B), loss of the outer fourth of the enamel blade on the posterior wall of M1 and M2 (also occurring only in the living species), development of a more pronounced heel on the M3 by progressive elongation of the posterior loph, reduction in size of the jugal and its displacement ventrally, which allows the maxillary and squamosal bones to meet along the dorsal border of the zygomatic arch. The last specialization is seen in at least one taxon of _Orthogeomys_ (_Orthogeomys cherriei costaricensis_). In my opinion, too much weight has been given to this feature in past classifications. Reduction of enamel in the upper dentition evidently occurred in the late Pleistocene, since the posterior plates on the upper cheek teeth were complete in specimens from the middle Pleistocene (_Z. persimilis_). Geomys