Type.—Thomomys Wied-Neuwied, 1839.
Chronologic and geographic range.—Known from late Pliocene (early Blancan) to Recent. Known primarily from western North America from southern Canada south to Central México in Pliocene, Pleistocene and Recent and in middle and late Pleistocene of Maryland and Florida.
Diagnosis.—Size small to medium (basilar length exclusive of T. bulbivorus, measuring from approximately 24 to 45, including both males and females); upper incisors without grooving, excepting fine, indistinct sulcus rarely near inner margin (grooving more common in T. monticola than in other Recent species); crowns of cheek teeth high, rooted and ever-growing; all molars, including M3, monoprismatic and anteroposteriorly compressed, sometimes (especially in subadults) having slight inflection on labial side in upper teeth and lingual side in lower teeth; molars bicolumnar in pre-final stages of wear (seen in juvenal teeth only), patterns of wear in both upper and lower molars resembling those of Pliosaccomys, except that crowns of m3 and M3 unite into single column in final stages of wear; enamel pattern interrupted in all cheek teeth, loss occurring only at sides of each column; transverse enamel blade completely covering posterior face of both P4 and p4; all upper and lower molars with two transverse enamel blades, one on anterior surface and one on posterior surface, of each tooth, including M3; small third plate sometimes persistent on broad side of tooth, labial side in upper molars and lingual side in lower molars (T. bulbivorus); skull generalized, neither unusually narrow and deep or broad and flat; usually without marked cresting or rugosity; masseteric ridge well developed and massive; basitemporal fossa absent, sometimes shallow depression forming in T. townsendii; pelage soft, never harsh or hispid, covering body with thick coat of hair; forefoot exceptionally small for fossorial mammal, claws not especially long; body form remarkably fossorial.
The tribe Thomomyini is monotypic, including only the genus Thomomys.
1839. Thomomys Wied-Neuwied, Nova Acta Phys. Med. Acad. Caesar. Leop.-Carol., 19(1):377.
1836. Oryctomys Eydoux and Gervais (in part), Mag. de Zool., 6:20, pl. 21. Type: Oryctomys (Saccophorus) bottae, from coast of California, probably near Monterey.
1903. Megascapheus Elliot, Field Columb. Mus., Publ. 76, Zool. Ser., 3(11):190, July 25. Type: Diplostoma bulbivorum Richardson, from Columbia River, probably near Portland, Ore.
1933. Pleisothomomys Gidley and Gazin, Jour. Mamm. 14:354. Type: Pleisothomomys potomacensis Gidley and Gazin, from Pleistocene, Cumberland Cave local fauna, Allegany County, Maryland.
Chronologic range.—Known from late Pliocene to Recent.