1936. Pliosaccomys Wilson, Carnegie Inst. Washington Publ., 473:20, May 21.
Type.—Pliosaccomys dubius Wilson, 1936, from Smiths Valley local fauna in Lyon County, Nevada.
Chronologic range.—Early Pliocene, late Clarendonian (Wolf Creek local fauna, South Dakota, and Nettle Springs local fauna, California) to Middle Pliocene, middle part of Hemphillian (Smiths Valley local fauna, Nevada, and McKay Reservoir and Otis Basin local faunas, Oregon).
Description.—Size small (alveolar length of mandibular tooth-row measuring 6.0 in holotype), about as in Thomomys monticola; upper incisor relatively broad and flat, having anterior face smooth, without trace of grooving; crowns of cheek teeth of medium height and rooted; enamel investment continuous and uninterrupted in all stages of wear; premolars permanently, biprismatic; P4 having anterior prism subtriangular and decidedly smaller that sub-crescentic posterior prism, and joined near centers by narrow, obliquely oriented isthmus; p4 having anterior prism subovate, posterior prism strongly compressed anteroposteriorly, and joined at mid-points by relatively broad and straight isthmus; first and second molars, both above and below, monoprismatic in final (?adult) stage of wear, derived ontogenetically from primitive bilophate pattern by coalescence of two columns into one; M1 and M2 mirror images of m1 and m2 in pre-final stages of wear, two columns first uniting at edge of protomeres forming U-pattern, and primitive H-pattern never developing in either series (for detailed description of stages of wear, see account of phylogeny, [p. 546]); m3 (M3 unknown, but probably with same form as in Geomyini, see [p. 552]) persistently biprismatic, two columns joined by relatively broad isthmus at centers, consequently, forming H-pattern of primitive ancestors; rostrum heavy and broad as in modern geomyids; palate narrow and strongly ribbed; mandible stout; masseteric ridge and fossa well developed; basitemporal fossa absent.
Specimens examined.—Six, nos. 1796 (holotype)—1799, 1804 and 1806 (CIT) now in the Los Angeles County Museum, all from Smiths Valley local fauna, Middle Pliocene, Nevada.
Referred species.—two:
*Pliosaccomys dubius Wilson, 1936. Carnegie Inst. Washington Publ., 743:20, May 21. Known from early and middle Pliocene faunas including Wolf Creek local fauna (late Clarendonian), Shannon County, South Dakota; McKay Reservoir local fauna and Otis Basin local fauna (Hemphillian), Oregon; type from Smiths Valley local fauna (probably middle Hemphillian), Lyon County, Nevada.
*Pliosaccomys wilsoni James, 1963. Univ. California Publ. Geol. Sci., 45:101, June 26. Type from Nettle Springs local fauna of late Clarendonian (early Pliocene), Ventura County, California.