Description.—Same as that given for the tribe Thomomyini above.
Discussion.—Features characterizing Thomomys and the tribe Thomomyini are more advanced than those characterizing the tribe Dikkomyini. Also, the Thomomyini retain more of the primitive features of the Geomyinae than do the more specialized tribe Geomyini.
Specializations are few, but include the third molar being a single column both above and below, enamel plates, and a masseteric ridge.
Key to the Subgenera of Thomomys
A Molars sub-crescent or ovate in cross-section, not becoming abruptly narrower at one end of tooth. Subgenus Pleisothomomys
A´ Molars pear-shaped, not sub-crescent or ovate, in cross-section, crown becoming abruptly narrow at one end of tooth. Subgenus Thomomys
Subgenus Pleisothomomys Gidley and Gazin
1933. Pleisothomomys Gidley and Gazin, Jour. Mamm., 14:354, November 13.