There is no record as yet of the early evolution of the subgenus _Cratogeomys_. The features that characterize the subgenus were already well developed in the first known fossils which are from Wisconsin deposits of the late Pleistocene. _Cratogeomys_ is not a homogenous assemblage; instead it is composed of two groups of living species, the generalized _castanops_ group and the specialized _gymnurus_ group. The _castanops_ group may be survivors of the ancestral lineage that diverged in two different stages in the phyletic development of the main line. Even so, the _castanops_ group has acquired its peculiar specializations. Indeed, _P. merriami_ of the _castanops_ group differs from the hypothetical stem more than does _P. castanops_. Judging from the structure of the living species of the subgenus _Cratogeomys_ and from the primitive subgenus _Pappogeomys_, the subgenus _Cratogeomys_ featured five major trends: (1) Increase in size; (2) formation of sagittal crest by union of the temporal impressions; (3) increase in rugosity and angularity of the skull; (4) progressive development of platycephalic specializations, including the elongation of the angular process of the mandible; (5) complete loss of enamel plates from the posterior wall of M1 and M2. Each trend is thought to be adaptive. [Illustration: FIG. 9. Molariform dentitions of the Tribe Geomyini. Drawings illustrating enamel patterns characteristic of _Geomys_ and _Pappogeomys_ (including the subgenera _Pappogeomys_ and _Cratogeomys_). × 5. A and B. _Geomys bursarius bursarius_, adult female, No. 46275 (KU), Elk River, Sherborne Co., Minnesota. Left upper (A), P4-M3; right lower (B), p4-m3. C and D. Subgenus _Pappogeomys_. _Pappogeomys bulleri albinasus_, adult female, No. 31002 (KU), W side La Venta, 13 mi. W and 4 mi. N Guadalajara, Jalisco. Left upper (C), P4-M3; right lower (D), p4-m3. E and F. Subgenus _Pappogeomys_. _Pappogeomys alcorni_, adult female, No. 31051 (KU), holotype, 4 mi. W Mazamitla, 6600 ft., Jalisco. Left upper (E), P4-M3; right lower (F), p4-m3. G and H. Subgenus _Cratogeomys_. _Pappogeomys gymnurus tellus_, adult female, No. 31051 (KU), 1 mi. NE Tala, 4400 ft., Jalisco. Left upper (G), P4-M3; right lower (H), p4-m3. ]