Loss of enamel is a trend common to all living genera of the tribe Geomyini, but the greatest loss has occurred in _Cratogeomys_. It has lost the plates on the posterior walls of M1 and M2 (Fig. 9G). If the lateral plates of M3 are considered as one functional plate and the lateral plates on either side of P4 together as two transverse plates, then, the transverse cutting blades in _Cratogeomys_ number seven in the upper and seven in the lower cheek teeth compared with 10 in the upper and seven in the lower in the primitive morphotype. Indeed, in some species of the subgenus, one or both of the lateral plates on M3 is also lost, usually in old age, resulting in even greater reduction of enamel. Loss of enamel from the posterior walls of the upper molars may be associated with changes in the mechanics of mastication from anteroposterior planing to anterotransverse shearing, as discussed elsewhere. Merriam (1895:95-96) argues convincingly that the posterior cutting blades of the upper molars would hinder efficient shearing action of the teeth; hence, selection would favor their reduction and eventual loss. Changes in the shape of the skull also seem to be correlated with the shift from a planing to a shearing type of mastication. More efficient shearing action, which depends upon lateral movement of the jaw, can be developed if the functional muscles insert farther laterally than is possible in the generalized type of skull. Therefore, platycephalic specializations involved lateral expansion of the braincase and mandible. Pronounced lateral expansion has been developed only in the _gymnurus_ group of species, suggesting that the dental specializations evolved earlier in the evolution of the subgenus than did the platycephalic specializations of the skull, and that the _castanops_ group separated from the _gymnurus_ group before the common ancestor had developed the more extreme trends in platycephaly. It is interesting to note that the subtriangular M3 (Fig. 9G) postulated for the ancestral morphotype and that characterizes the subgenus _Pappogeomys_ is retained also in the _gymnurus_ group. LITERATURE CITED ALSTON, E. R. 1876. On the classification of the order Glires. Proc. Zool. Soc. London, 1876:61-98, 1 pl., June. ALVAREZ, T. 1964. Nota sobre restos oseos de mamiferos del Reciente, encontrados cerca de Tepeapulco, Hidalgo, Mexico. Publ. Inst. Nac. Antro, e Hist., 15:1-15. 1965. Catálago Paleomastozoológico Mexicano. Publ. Inst. Nac. Antro, e Hist., 17:1-70. AXLEROD, D. I. 1950. Studies in late Tertiary paleobotany. Carnegie Inst. Washington Publ., 590:1-322. 1958. Evolution of the Madro-tertiary geoflora. Bot. Rev., 24:433-509. BADER, R. S., and TECHTER, D. 1959. A list and bibliography of the fossil mammals of Illinois. Nat. Hist. Misc., Chicago Acad. Sci., 172:1-8, October 30. BAIRD, S. F.