Part 6

_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_.

Genus =Thomomys= Wied-Neuwied

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.

_Description._--Same as that given for the tribe Thomomyini above.

_Discussion._--Features characterizing _Thomomys_ and the tribe Thomomyini are more advanced than those characterizing the tribe Dikkomyini. Also, the Thomomyini 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 masseteric ridge.

Key to the Subgenera of _Thomomys_

A Molars sub-crescent or ovate in cross-section, not becoming abruptly narrower at one end of tooth. Subgenus _Pleisothomomys_ p. 519

A´ Molars pear-shaped, not sub-crescent or ovate, in cross-section, crown becoming abruptly narrow at one end of tooth. Subgenus _Thomomys_ p. 520

Subgenus =Pleisothomomys= Gidley and Gazin

1933. _Pleisothomomys_ Gidley and Gazin, Jour. Mamm., 14:354, November 13.

_Type._--_Pleisothomomys 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 assemblage that probably was deposited in the Sangamon. The middle and late Pleistocene records are from the eastern United States, suggesting that the subgenus _Pleisothomomys_ was restricted to that region while the subgenus _Thomomys_ occupied the western United States and parts of Canada and México as it does today.

_Description and Comparison._--Separated from subgenus _Thomomys_ 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 _Thomomys vetus_ of the late Pleistocene (Wisconsin deposits), referred to the subgenus _Thomomys_ on the basis of its alleged relationship to _Thomomys 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 _Thomomys_. _Pleisothomomys_ 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 _Thomomys_ occurring in Pleistocene times in the eastern United States.

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

*_Thomomys gidleyi_ Wilson, 1933. Carnegie Inst. Washington Publ. 440:122, December. Type from Hagerman beds, late Pliocene, Idaho.

*_Thomomys potomacensis_ Gidley and Gazin, 1933. Jour. Mamm., 14:354, November 13. Type from Cumberland Cave, middle and late Pleistocene, Maryland.

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

Subgenus =Thomomys= Wied-Neuwied

1839. _Thomomys_ 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._--_Thomomys 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 _Thomomys vetus_ are less distinctly pear-shaped than other referred species (see remarks in the description of the subgenus _Pleisothomomys_).

Essentially on the basis of its significantly larger size and details of the skull, Elliott (1903:190) proposed subgeneric recognition of _Thomomys bulbivorus_ and described the subgenus _Megascapheus_ to include it. Also the molars of _Thomomys 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 _Thomomys 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

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

_umbrinus_ species-group

*_Thomomys scudderi_ Hay, 1921. Proc. U. S. Nat. Mus., 49:614. Type from Fossil Lake beds, late Pleistocene, Oregon.

_Thomomys umbrinus_ (Richardson, 1829). Fauna Boreali-Americana, 1:202. Type from southern México, probably near Boca de Monte, Veracruz.

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

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

_Thomomys 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

*_Thomomys microdon_ Sinclair, 1905. Bull. Dept. Geol. Univ. California, 4:145-161. Type from Potter Creek Cave, late Pleistocene, California.

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

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

_Thomomys 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 Panamá, and probably northern Colombia, South America.

_Diagnosis._--Size small to large (condylobasal length of skull 33.0 to 73.0 in adults, including both sexes); sexual 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; masseteric ridge and fossa weakly developed in primitive members, becoming well developed and massive 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 massive, 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; masseteric ridge weakly developed, low, not massive. 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); masseteric crest strongly developed and massive.

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 labial 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 massive; length of lingual plate of M3 approximately equal to, or greater than, length of labial 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 labial 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

Genus =Pliogeomys= Hibbard

1954. _Pliogeomys_ Hibbard, Michigan Acad. Sci., Arts and Letters, 39:353.

_Genotype._--_Pliogeomys buisi_ Hibbard, 1954, from Buis Ranch local fauna (middle Pliocene), Beaver County, Oklahoma.

_Chronologic range._--Latest Middle Pliocene, known only from the highest part of the Hemphillian mammalian fauna (Buis Ranch local fauna, Oklahoma). Professor Hibbard informs me (personal communication) that he found the type, a right ramus, lying on the surface near the base of the fossil beds. The isolated teeth of small geomyids from the Saw Rock Canyon local fauna (see Hibbard, 1953:392) may also be referable to this genus. The Saw Rock Canyon local fauna may also be middle Pliocene in age but is considered to be from the later part of the late Pliocene, and, therefore, somewhat younger than the Buis Ranch local fauna (Hibbard, _op. cit._:342).

_Description and discussion._--The size of members of this small genus of the Geomyinae is about the same as in smaller adults of _Geomys bursarius_. According to Hibbard (_op. cit._:353), the holotype is smaller than specimens from the Rexroad local fauna referred to _Geomys quinni_ and larger than specimens referred to _Zygogeomys_ cf. _minor_. The cheek teeth are rooted, and the crowns are as high as those of living geomyids. The upper incisor is bisulcate, and the inner groove is fine and indistinct in places.

Of the molariform dentition only the lower premolar and first two lower molars are known. The enamel investment of p4 is complete, and would not be subject to interruption at any stage of wear; the two prisms are joined at their mid-points, and the isthmus of dentine is relatively broad (as in _Pliosaccomys_) when compared with modern pocket gophers of this tribe. Also, the re-entrant folds, rather than having parallel sides, diverge broadly to the sides. The divergence is especially noticeable in the labial fold. The lower deciduous premolar would have formed essentially the same enamel pattern with wear as observed in _Nerterogeomys_ [= _Zygogeomys_] cf. _minor_ (see Hibbard, 1954:fig. 5, A and B) and _Pliosaccomys dubius_ (see Wilson, 1936; pl. 1, fig. 1). Each molar is a single column in the final stages of wear; pre-final stages are unknown. Anterior and posterior enamel plates are present on m1 and m2 (m3 has not been recovered). The dentine tracts of m1 are exposed over a relatively wide surface; therefore, the enamel plates are distinctly separated. The tracts of dentine of m2 are much narrower than in m1 and the enamel plates are barely separated at the anterolateral margin of the tooth. Possibly the enamel band of m2 was continuous in an earlier stage of wear.

The mandible is stout and its general construction not unlike that in modern geomyines. The capsule at the base of the angular process that receives the terminal end of the lower incisor is well developed. The base of the angular processes is preserved, and suggests that the process was short and decidedly smaller than in living examples of the tribe. The masseteric ridge is distinct but weakly developed, and not at all massive as in living pocket gophers. The mental foramen is immediately anterior, and slightly ventral, to the anterior extension of the crest. The basitemporal fossa is absent as such, but its position is marked by a slight depression.

_Specimens examined._--Two rami; nos. 29147 (holotype) and 33446; several isolated teeth 30194 and 30195, including an upper incisor and a dp4 (deciduous lower premolar), all from Univ. Michigan Mus. Paleo.

_Referred species._--One.

*_Pliogeomys buisi_ Hibbard, 1954. Papers Michigan Acad. Sci., Arts, and Letters, 39:353. Type from Buis local fauna, latest middle Pliocene, Beaver County, Oklahoma.

Genus =Zygogeomys= Merriam

1895. _Zygogeomys_ Merriam, N. Amer. Fauna, 8:195, January 31.

1942. _Nerterogeomys_ Gazin, Proc. U. S. Nat. Mus., 92:507 (type, _Geomys persimilis_ Hay, 1927).

_Type._--_Zygogeomys trichopus_ Merriam, 1895, from Nahuatzen, Michoacán.

_Chronologic range._--Late Pliocene (Benson and Curtis Ranch local faunas, Arizona, and ?Rexroad Formation, Kansas) to Recent.

_Description and discussion._--The size is small to medium for the subfamily Geomyinae. This genus is distinguished principally by the retention of primitive features. In the living species, the skull is generalized, rather than specialized toward either extreme dolichocephaly or platycephaly. The angular process is short, barely exceeding the lateral extensions of the mastoid process of the squamosal. The rostrum is remarkably narrow in relation to its length. The jugal is reduced and displaced ventrally, causing the maxillary arm of the zygomata to articulate with the squamosal arm of the zygomata along the dorsal border of the zygomatic arch (a feature observed also in _Orthogeomys cherriei costaricensis_).

The upper incisor, recovered in material from the late Pliocene and middle Pleistocene, is bisulcate as in the genus _Geomys_ and the primitive genus _Pliogeomys_. The enamel plate across the posterior wall of P4 is either complete (late Pliocene to late Pleistocene) or restricted to the lingual half of the tooth (always restricted in living species). The Pliocene specimens of the Rexroad local fauna referred to _Nerterogeomys_ cf. _minor_ by Hibbard (1950:138-139) are exceptional. In these specimens the length and position of the posterior enamel plate is variable; however, all but one specimen had persistant enamel. Evidently, in approximately 43 per cent of the specimens, a complete enamel blade was present (see Paulson, 1961:139), and in the others (except the one without any enamel) the plate was restricted to a small area of the ventral surface, usually on the lingual side of the loph. Hibbard suggested that the decrease in size of the plate, and its restriction to the lingual side, may be a function of age. Hence, most adults would be characterized by the reduced posterior plate on the upper premolar. Although age may be the important factor, intragroup variation cannot be ruled out. It is of interest to note that in all specimens from the Benson (type series of _P. minor_) and Curtis Ranch local faunas, the former of late Pliocene age and the latter of middle Pleistocene age, the enamel plates are complete on the posterior face of the upper premolar. As mentioned before, the specimens from Kansas may actually represent the transitional stages of the early evolution of _Geomys_ in which the posterior plate of P4 is entirely lost. The enamel pattern of p4 is like that in other members of the tribe (excepting the genus _Pliogeomys_). The re-entrant angles of P4 and p4 are widely open (obtuse) in the examples recovered from late Pliocene and middle Pleistocene deposits, representing retention of a trait that is primitive in the Geomyini (see account of phylogeny).

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_).