Key to the Genera of the Tribe Dikkomyini

A Molars biprismatic throughout life; anterior and posterior lophs of first and second molars in pre-final stages of wear uniting first at their mid-points and later at edge of protomeres; anterior lophid of lower premolar having distinct anteroexternal inflection. Genus _Dikkomys_ p. 516

A' First and second molars becoming monoprismatic in final (adult?) stages of wear, biprismatic only in pre-final stages of wear; third molars persistently biprismatic; anterior and posterior lophs of first and second molars uniting first at edge of protomeres; anterior lophid of lower premolar lacking anteroexternal inflection. Genus _Pliosaccomys_ p. 517

Genus =Dikkomys= Wood

1936. _Dikkomys_ Wood, Amer. Mus. Novit., 866:26, July 2.

_Type._--_Dikkomys matthewi_ Wood, 1936, from Lower Harrison deposits near Agate, Sioux County, Nebraska.

_Chronologic range._--Early Miocene, from early Arikareean (Lower Harrison local fauna of Nebraska) to middle Miocene, late Hemingfordian (Upper Rosebud local fauna, South Dakota, and the Deep River Formation, Montana). According to MacDonald (1963:149-150), the Upper Rosebud is middle Miocene rather than early Miocene.

_Description._--Size small, about as in small kinds of _Th.o.m.omys_; known only from fragmentary mandible, including molariform dent.i.tion in place, and isolated cheek teeth, including M1 (see Wood, 1936:26-28 and fig. 32; Galbreath, 1948:316-317 and fig. 1; and Black, 1961:13-14 and fig. 58); upper incisors unknown; cheek teeth hyposodont, persistently rooted, and having crowns of medium height compared with Recent geomyids; enamel investment complete and uninterrupted in all molariform teeth in all stages of wear; P4 unknown, but probably formed like p4; p4 persistently biprismatic, two crowns joined at mid-points by relatively narrow isthmus separating lateral re-entrant folds; anterior lophid of p4 having distinct anteroexternal inflection; molars also biprismatic throughout life; two lophids of lower molars first uniting at mid-points as in p4, and, with additional wear, m1 and m2 secondarily uniting at edge of protomeres and forming isolated enamel fossette between point of connection (detailed description of stages of wear discussed in account of phylogeny of subfamily); m3 permanently joined at mid-point only, without lateral union at edge of protomeres; upper molars, judging by M1 (M2 and M3 unknown), having same pattern as lower molars, but first union of lophs decidedly on lingual side of center, consequently, lingual re-entrant fold small; M1 probably developing U-pattern in advanced stages of wear by union of protomeres, with minute lingual fossette developing in transition as lophs secondarily become united at lingual edge of columns; mandible stout and geomyidlike; ma.s.seteric ridge weakly developed; basitemporal fossa absent.

Evidently, _Dikkomys matthewi_ is more primitive than _Dikkomys woodi_. The modified H-pattern in m1 and m2, with the metalophid and hypolophid joined at both their mid-points and also at their protomeres (by union of the protostylid and hypostylid in the lower dent.i.tion), is persistent throughout life. Therefore, the enclosed enamel fossette is not eradicated with wear. In m1 and m2 of _Dikkomys woodi_, the fossette is shallower, and, at least in advanced stages of wear, it would disappear, therefore, forming a U-pattern on the occlusal surface, as in M1 and M2, but lateral inflection horizontally shallow rather than deep as in entoptychines.

Specimen (No. P 26284 FMNH) reported as _Dikkomys matthewi_ by Galbreath (1948:316) is referable to the recently described species _Dikkomys woodi_ Black, 1961.

_Specimens examined._--One, no. P 26284, Field Mus. Nat. Hist., from upper Rosebud, Shannon Co., South Dakota.

_Referred species._--two:

_Dikkomys matthewi_ Wood, 1936. Amer. Mus. Novit., 866:26, July.

Type from early Arikareean Lower Harrison deposits (early Miocene) near Agate, Sioux County, Nebraska.

_Dikkomys woodi_ Black, 1961. Postilla, Yale Peabody Museum, 48:13, January 16. Type from Deep River Formation, late Hemingfordian (middle Miocene), Meagher County, Montana; also known from Upper Rosebud deposits (middle Miocene) near Wounded Knee, Shannon County, South Dakota.

Genus =Pliosaccomys= Wilson

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 _Th.o.m.omys 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; ma.s.seteric 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.

Tribe Th.o.m.oMYINI, new tribe

_Type._--_Th.o.m.omys_ 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 Mexico 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 l.a.b.i.al 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, l.a.b.i.al 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; ma.s.seteric ridge well developed and ma.s.sive; 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 Th.o.m.omyini is monotypic, including only the genus _Th.o.m.omys_.

Genus =Th.o.m.omys= Wied-Neuwied

1839. _Th.o.m.omys_ 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. _Pleisoth.o.m.omys_ Gidley and Gazin, Jour. Mamm. 14:354. Type: _Pleisoth.o.m.omys potomacensis_ Gidley and Gazin, from Pleistocene, c.u.mberland Cave local fauna, Allegany County, Maryland.

_Chronologic range._--Known from late Pliocene to Recent.

_Description._--Same as that given for the tribe Th.o.m.omyini above.

_Discussion._--Features characterizing _Th.o.m.omys_ and the tribe Th.o.m.omyini are more advanced than those characterizing the tribe Dikkomyini. Also, the Th.o.m.omyini 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 ma.s.seteric ridge.

Key to the Subgenera of _Th.o.m.omys_

A Molars sub-crescent or ovate in cross-section, not becoming abruptly narrower at one end of tooth.

Subgenus _Pleisoth.o.m.omys_ p. 519

A' Molars pear-shaped, not sub-crescent or ovate, in cross-section, crown becoming abruptly narrow at one end of tooth. Subgenus _Th.o.m.omys_ p. 520

Subgenus =Pleisoth.o.m.omys= Gidley and Gazin

1933. _Pleisoth.o.m.omys_ Gidley and Gazin, Jour. Mamm., 14:354, November 13.

_Type._--_Pleisoth.o.m.omys potomacensis_ Gidley and Gazin, 1933.

_Chronologic range._--Late Pliocene (Hagerman local fauna, Idaho) to late Pleistocene. The latest records are from the fauna of Saber-tooth Cave, Florida, a late Pleistocene a.s.semblage that probably was deposited in the Sangamon. The middle and late Pleistocene records are from the eastern United States, suggesting that the subgenus _Pleisoth.o.m.omys_ was restricted to that region while the subgenus _Th.o.m.omys_ occupied the western United States and parts of Canada and Mexico as it does today.