M1 and M2 are elliptical in cross-section and each has an enamel plate on both the anterior and posterior surface. In the living species (_Z. trichopus_), the posterior enamel plate fails to reach the labial margin of the tooth and is restricted to the lingual two-thirds of the posterior surface; however, the enamel plates are complete in the late Pliocene species (_Z. minor_) and the middle Pleistocene species (_Z. persimilis_), being only slightly separated from the anterior plate by narrow tracts of dentine on the ends of the tooth. M3 is partly biprismatic in the living species, the two incompletely divided lophs being separated by a distinct outer sulcus. The posterior loph is elongated and forms a conspicuous heel paralleling the evolution of this trait in the genus _Orthogeomys_; therefore, the crown is longer than wide. The posterior part of the tooth is protected by two lateral enamel plates; of the two, the lingual plate is especially long and extends to the end of the heel. M3 has not been recovered in the Pliocene species, but in the middle Pleistocene species (_Z. persimilis_) M3 is subtriangular, no longer than wide, and the lateral inflections are weakly developed. The trend towards elongation of M3 evidently occurred in late Pleistocene evolution of the genus. All three of the inferior molars are elliptical, and only the posterior enamel plate is present (as in all other genera of the tribe except _Pliogeomys_). The masseteric ridge of the mandible is well developed. In the late Pliocene species _Z. persimilis_ and _Z. minor_ the mental foramen is directly beneath the anterior extension of the masseteric ridge, but in the living species, _Z. trichopus_, the foramen lies well anterior to the ridge. The basitemporal fossa in the living species is well developed and deep; in the Pliocene species it is usually distinct but shallow (late Pliocene specimens of _Z. minor_). _Referred species._--Three (two extinct and one living; the last has two subspecies): *_Zygogeomys minor_ (Gidley), 1922. U. S. Geol. Surv. Prof. Paper, 131:123, December 26. Type from Benson local fauna (late Pliocene), Cochise County, Arizona; also known from the Rexroad local fauna, Meade County, Kansas. *_Zygogeomys persimilis_ Hay, 1927. Carnegie Inst. Washington Publ., 136. Originally described by Gidley, 1922 (U. S. Geol. Surv. Prof. Papers, 131:123, December 26) as _Geomys parvidens_ which was preoccupied by _G. parvidens_ Brown, 1908. Type from Curtis Ranch local fauna (middle Pleistocene), Cochise County, Arizona. _Zygogeomys trichopus trichopus_ Merriam, 1895. N. Amer. Fauna, 8:196, January 31. Type from Nahuatzen, Michoacán. _Zygogeomys trichopus tarascensis_ Goldman, 1938. Proc. Biol. Soc. Washington, 51:211, December 23. Type from 6 mi. SE Pátzcuaro, 8,000 ft., Michoacán. Genus =Geomys= Rafinesque