_Description and Comparison._--Separated from subgenus _Th.o.m.omys_ only on basis of sub-crescentic shaped molars (only jaw fragments and isolated teeth known), seemingly a primitive feature of the genus.

This dental structure continued into the late Pleistocene; none of the Recent species expresses this feature of the molars, although the molars of _Th.o.m.omys vetus_ of the late Pleistocene (Wisconsin deposits), referred to the subgenus _Th.o.m.omys_ on the basis of its alleged relationship to _Th.o.m.omys townsendii_ (see Davis, 1937:156-158), are less distinctly pear-shaped, and are more sub-crescentic, than in any other known species of the subgenus _Th.o.m.omys_. _Pleisoth.o.m.omys_ Gidley and Gazin (_loc. cit._) was proposed as a genus but is here considered as of no more than subgeneric worth, and is recognized because of the apparent constancy of the sub-crescentic molars in the earlier members of the genus and in those populations of _Th.o.m.omys_ occurring in Pleistocene times in the eastern United States.

_Referred species._--Three (all extinct):

*_Th.o.m.omys gidleyi_ Wilson, 1933. Carnegie Inst. Washington Publ.

440:122, December. Type from Hagerman beds, late Pliocene, Idaho.

*_Th.o.m.omys potomacensis_ Gidley and Gazin, 1933. Jour. Mamm., 14:354, November 13. Type from c.u.mberland Cave, middle and late Pleistocene, Maryland.

*_Th.o.m.omys orientalis_ Simpson, 1928. Amer. Mus. Novit., 328:6, October 26. Type from Saber-tooth Cave, late Pleistocene, Florida.

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

1839. _Th.o.m.omys_ Wied-Neuwied, Nova Acta Phys.-Med. Acad. Caesar.

Leop. Carol., 19(1):377.

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, Oregon.

_Type._--_Th.o.m.omys rufescens_ Wied-Neuwied, 1839.

_Chronologic range._--Early Pleistocene (Broadwater-Lisco local fauna, Nebraska) to Recent. Numerous records, mostly isolated teeth, from nearly all stratigraphic levels of the Pleistocene (for details, see account of fossil record).

_Description._--Molars pear-shaped in cross-section, becoming abruptly narrow at one end of the tooth. The teeth of the late Pleistocene species _Th.o.m.omys vetus_ are less distinctly pear-shaped than other referred species (see remarks in the description of the subgenus _Pleisoth.o.m.omys_).

Essentially on the basis of its significantly larger size and details of the skull, Elliott (1903:190) proposed subgeneric recognition of _Th.o.m.omys bulbivorus_ and described the subgenus _Megascapheus_ to include it. Also the molars of _Th.o.m.omys bulbivorus_ usually have a small enamel plate, both above and below, bordering the persistent inflection on the protomere end of the tooth; each lateral plate is isolated from the transverse plates on the anterior and posterior walls of the tooth. In my opinion these features do not warrant subgeneric recognition; however, these characters do distinctly separate _Th.o.m.omys bulbivorus_ from other groups of species, and the character of the molars suggests retention of a primitive trait.

Therefore, I propose that the unique structure of this species be recognized by setting it apart in the _bulbivorus_ species-group.

_Referred species._--Ten species, three extinct, placed in three species-groups (the numerous subspecies of this genus are listed in Miller and Kellogg, 1955:276-332, and Hall and Kelson, 1959:412-447).

_bulbivorus_ species-group

_Th.o.m.omys bulbivorus_ (Richardson, 1829). Fauna Boreali-Americana, 1:206. Type from Columbia River, probably near Portland, Oregon.

_umbrinus_ species-group

*_Th.o.m.omys scudderi_ Hay, 1921. Proc. U. S. Nat. Mus., 49:614.

Type from Fossil Lake beds, late Pleistocene, Oregon.

_Th.o.m.omys umbrinus_ (Richardson, 1829). Fauna Boreali-Americana, 1:202. Type from southern Mexico, probably near Boca de Monte, Veracruz.

_Th.o.m.omys bottae_ (Eydoux and Gervais, 1836). Mag. de Zool., Paris, 6:23. Type from coast of California, probably near Monterey.

*_Th.o.m.omys vetus_ Davis, 1937. Jour. Mamm., 18:156, May 12. Type from Fossil Lake beds, late Pleistocene, Oregon.

_Th.o.m.omys townsendii_ (Bachman, 1839). Jour. Acad. Nat. Sci.

Philadelphia, 8:105. Type probably from near Nampa, Canyon Co., Idaho (erroneously given as "Columbia River").

_talpoides_ species-group

*_Th.o.m.omys microdon_ Sinclair, 1905. Bull. Dept. Geol. Univ.

California, 4:145-161. Type from Potter Creek Cave, late Pleistocene, California.

_Th.o.m.omys monticola_ J. A. Allen, 1893. Bull. Amer. Mus. Nat. Hist., 5:48, April 28. Type from Mt. Tallac, 7500 ft., El Dorado Co., California.

_Th.o.m.omys talpoides_ (Richardson, 1828). Zool. Jour., 3:518. Type locality fixed at near Fort Carlton (Carlton House), Saskatchewan River, Saskatchewan, Canada.

_Th.o.m.omys mazama_ Merriam, 1897. Proc. Biol. Soc. Washington, 11:214, July 15. Type from Anna Creek, 6000 ft., near Crater Lake, Mt. Mazama, Klamath Co., Washington.

Tribe GEOMYINI, new tribe

_Genotype._--_Geomys_ Rafinesque, 1817.

_Chronologic and geographic range._--Known from late middle Pliocene deposits to Recent. The range of living members extends from extreme southern Manitoba and the southeastern United States south to southern Panama, and probably northern Colombia, South America.

_Diagnosis._--Size small to large (condylobasal length of skull 33.0 to 73.0 in adults, including both s.e.xes); s.e.xual dimorphism marked, sometimes strongly, females being smaller than males, especially in cranial dimensions; upper incisors invariably grooved, number and position of grooves varying according to genus; cheek teeth high-crowned and ever-growing, except in one primitive genus (_Pliogeomys_); all three lower molars and M1 and M2 monoprismatic, and elliptical in cross-section in final stages of wear (teeth of young, subadult, and adult animals); primitive biprismatic patterns (as known from Recent specimens) occurring only in pre-final stages of wear (teeth of juveniles only); biprismatic patterns of lower molars as in _Dikkomys_, and upper molars as in _Pliosaccomys_ (for detailed description of these patterns, see account beyond of the phylogeny of the Geomyinae); m3 becoming monoprismatic, anteroposteriorly compressed and elliptical in cross-section like m1 and m2, but M3 remaining, with rare exceptions (see accounts of _Geomys_ and _Pappogeomys_ beyond), at least partially biprismatic throughout life, having one or both lateral inflections usually persisting (with exceptions) and developing various occlusal shapes (subtriangular, elongate, obcordate, suborbiculate, or quadriform) but never elliptical.

Enamel of cheek teeth reduced to interrupted plates, with exception of p4 in _Pliogeomys_; plate on posterior wall of P4 variable, occurring completely across posterior surface in primitive members, but progressively reduced to lingual side only or completely lost in modern genera (see generic accounts beyond for detailed description); both anterior and posterior plates usually retained in M1 and M2, posterior plate sometimes reduced to lingual side or completely lost (as in _Pappogeomys_) but anterior plate always completely retained; M3 usually having three plates, one anterior and two lateral; posterior plate wanting (sometimes lingual plate moved to posterior position); plates retained completely across posterior walls of all lower cheek teeth with no reduction, but anterior plates of m1-3 always lacking, except in primitive genus _Pliogeomys_ (only Geomyini having both anterior and posterior enamel plates on lower molars).

Skull primitively generalized, but becoming specialized towards either dolichocephaly (_Orthogeomys_) or platycephaly (_Pappogeomys_) in two modern genera; skull highly specialized for fossorial life; mandible stout and deep, angular process being high and diverging laterally at right angles to ramus; ma.s.seteric ridge and fossa weakly developed in primitive members, becoming well developed and ma.s.sive in modern genera; basitemporal fossa absent in primitive forms (_Pliogeomys_ and early members of _Zygogeomys_); pelage usually soft, but harsh and hispid in some genera; forefeet broad and ma.s.sive, claws long and stout for digging; body form remarkably fossorial.

The tribe Geomyini includes the most highly specialized members of the subfamily Geomyinae.

Key to the Genera of the Tribe Geomyini

A Cheek teeth rooted; p4 with uninterrupted enamel loop; enamel plates on both anterior and posterior walls of m1 and m2; ma.s.seteric ridge weakly developed, low, not ma.s.sive.

Genus _Pliogeomys_ p. 522

A' Cheek teeth rootless, ever-growing; p4 with enamel investment interrupted at ends of columns, consequently, forming four isloted plates; enamel plate retained only on posterior wall of m1 and m2, anterior wall without trace of enamel (except rarely in pre-final stage of wear in _Geomys tobinensis_ of middle Pleistocene); ma.s.seteric crest strongly developed and ma.s.sive.

B Enamel plate on posterior wall of P4, but usually restricted to lingual end of tooth (usually absent in subgenus _Orthogeomys_ of genus _Orthogeomys_); M3 conspicuously bicolumnar, longer than wide owing to elongation of posterior loph.

C Upper incisor bisulcate; skull generalized; rostrum relatively narrow; length of l.a.b.i.al enamel plate of M3 decidedly less than length of lingual plate; pelage soft and thick. Genus _Zygogeomys_ p. 523

C' Upper incisor unisulcate; skull strongly dolichocephalic; rostrum remarkably broad and ma.s.sive; length of lingual plate of M3 approximately equal to, or greater than, length of l.a.b.i.al plate; pelage harsh, often hispid and scant. Genus _Orthogeomys_ p. 528

B' Posterior wall of P4 without trace of enamel; M3 not strongly bicolumnar, having shallow re-entrant fold on l.a.b.i.al side, and crown no longer than wide owing to shortness of posterior loph.

D Upper incisor bisulcate; skull generalized; both anterior and posterior walls of M1 and M2 having complete enamel plates. Genus _Geomys_ p. 525

D' Upper incisor unisulcate; skull generalized or tending towards platycephaly; enamel plate on posterior wall of M1 usually reduced to lingual side or absent (complete only in one species, _Pappogeomys bulleri_); enamel plate on posterior wall of M2 also absent in advanced species (subgenus _Cratogeomys_). Genus _Pappogeomys_ p. 532