The variable character of the third upper molar as between species suggests that this tooth is presently undergoing active evolution. The structure of this tooth, although differing between taxa, is remarkably stable in other kinds of Geomyini. The most remarkable modification of M3 in _Cratogeomys_ is the obcordate pattern developed in _P. merriami_ of the _castanops_-group. The posterior loph and entire tooth is shortened somewhat resembling in shape that of _Thomomys_. Moreover, the posterior loph is twisted labially; consequently, its posterior surface now forms the labial border of the weakly defined posterior loph. Owing to the torsion, the lingual enamel plate has been rotated to the posterior surface of the tooth. Therefore, the tooth is provided with two transverse enamel plates, including the plate on the anterior wall of the tooth. The labial plate is greatly reduced, its total surface being restricted to the small labial inflection. The highly specialized obcordate M3 is not found in the most specialized platycephalic skulls characteristic of the _gymnurus_ species-group. Instead the _gymnurus_-group retains the primitive subtriangular pattern without significant modification. _Referred species._--Seven: _castanops_ species-group _Pappogeomys castanops_ (Baird, 1852). Report Stanbury's Exp'd. to Great Salt Lake, p. 313, June. Type from "Prairie road to Bent's Fort," near present town of Las Animas, Colorado. _Pappogeomys merriami_ (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6, 12:271, October. Type from "southern Mexico," probably Valley of México (see Merriam, 1895:152). _gymnurus_ species-group _Pappogeomys fumosus_ (Merriam, 1892). Proc. Biol. Soc. Washington, 7:165, September 29. Type from 3 mi. W Colima, Colima. _Pappogeomys gymnurus_ (Merriam, 1892). Proc. Biol. Soc. Washington, 7:166, September 29. Type from Zapotlan (Ciudad Guzman), Jalisco. _Pappogeomys neglectus_ (Merriam, 1902). Proc. Biol. Soc. Washington, 15:68, March 22. Type from Cerro de la Calentura, about 8 mi. NW Pinal de Amoles, Querétaro. _Pappogeomys tylorhinus_ (Merriam, 1895). N. Amer. Fauna, 8:167, January 31. Type from Tula, Hidalgo. _Pappogeomys zinseri_ (Goldman, 1939). Jour. Mamm., 20:91, February 15. Type from Lagos, Jalisco. PHYLOGENY OF THE GEOMYIDAE