Chapter III: Part 3

_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_).

The masseteric ridge of the mandible is well developed. In the late Pliocene species _Z. persimilis_ and _Z. minor_ the mental foramen is directly beneath the anterior extension of the masseteric ridge, but in the living species, _Z. trichopus_, the foramen lies well anterior to the ridge. The basitemporal fossa in the living species is well developed and deep; in the Pliocene species it is usually distinct but shallow (late Pliocene specimens of _Z. minor_).

_Referred species._--Three (two extinct and one living; the last has two subspecies):

*_Zygogeomys minor_ (Gidley), 1922. U. S. Geol. Surv. Prof. Paper,
131:123, December 26. Type from Benson local fauna (late
Pliocene), Cochise County, Arizona; also known from the
Rexroad local fauna, Meade County, Kansas.

*_Zygogeomys persimilis_ Hay, 1927. Carnegie Inst. Washington
Publ., 136. Originally described by Gidley, 1922 (U. S. Geol.
Surv. Prof. Papers, 131:123, December 26) as _Geomys
parvidens_ which was preoccupied by _G. parvidens_ Brown,
1908. Type from Curtis Ranch local fauna (middle
Pleistocene), Cochise County, Arizona.

_Zygogeomys trichopus trichopus_ Merriam, 1895. N. Amer. Fauna,
8:196, January 31. Type from Nahuatzen, Michoacán.

_Zygogeomys trichopus tarascensis_ Goldman, 1938. Proc. Biol. Soc.
Washington, 51:211, December 23. Type from 6 mi. SE
Pátzcuaro, 8,000 ft., Michoacán.

Genus =Geomys= Rafinesque

1817. _Geomys_ Rafinesque, Amer. Monthly Mag., 2(1):45, November.

1817. _Diplostoma_ Rafinesque, Amer. Monthly Mag., 2(1):44-45,
November. Included species: _Diplostoma fusca_ Rafinesque
[= _Mus bursarius_ Shaw] and _Diplostoma alba_ Rafinesque
[= _Mus bursarius_ Shaw] from the Missouri River region.

1820. _Saccophorus_ Kuhl, Beitr. Zool. und Vergl. Anat., pp. 65, 66.
Type: _Mus bursarius_ Shaw, from upper Mississippi Valley.

1823. _Pseudostoma_ Say, Long's Expd. Rocky Mts., I, pp. 406. Type:
_Pseudostoma bursaria_ [= _Mus bursarius_ Shaw], from upper
Mississippi Valley.

1825. _Ascomys_ Lichtenstein, Abh. K. Akad. Wiss. Berlin (1822),
p. 20., fig. 2. Type: _Ascomys canadensis_ Lichtenstein
[= _Mus bursarius_ Say], probably from upper Mississippi Valley.

1944. _Parageomys_ Hibbard, Bull. Geol. Soc. Amer., 55:735, June.
Type: _Parageomys tobinensis_ Hibbard, from Pleistocene, Cudahy
(Tobin) local fauna, Russell Co., Kansas.

_Type._--_Geomys pinetis_ Rafinesque, 1817, restricted to Screven County, Georgia, in region of the pines.

_Chronologic range._--Late Pliocene faunas of Blancan age (Rexroad, Kansas, and Sand Draw, Nebraska, local faunas) to Recent. Reported from numerous Pleistocene deposits of all stratigraphic levels, especially from the Great Plains, where common today.

_Description and discussion._--Pocket gophers of this genus are medium-sized geomyids; none is so small as the average-sized _Thomomys_. The skull is generalized and lacks the dolichocephalic and platycephalic specializations seen in the genera _Orthogeomys_ and _Pappogeomys_, respectively. _Geomys_ closely resembles _Zygogeomys_, but retains fewer of the primitive characters of the ancestral stock. At the same time, _Geomys_ has several specializations. Even so, a considerable amount of parallelism is evident in the phyletic trends of the two genera.

The upper incisor of _Geomys_ is bisulcate as in _Pliogeomys_ and _Zygogeomys_; the deeper grove is medial and the shallower grove lies near the inner border of the tooth. The premolar, above and below, is bicolumnar; and two columns are joined at their mid-points (deep re-entrant angles separate the columns at the sides). A permanent enamel plate protects the anterior face of the anterior loph, and enamel bands outline each of the re-entrant folds. In p4 a complete enamel plate covers the posterior surface of the posterior loph. All of the enamel bands are interrupted by tracts of dentine, except in the initial stages of wear of the occlusal surface of the newly erupted tooth. For a short time in living _Geomys_, the enamel bands are continuous as observed in juveniles of _Geomys bursarius major_ (KU 5628, 8531, and 41540). But, the enamel cap is thin and the dentine tracts, which are high on the sides of the tooth, are soon revealed by a minimum of wear on the crown. Therefore, the adult, or final, pattern characterized by interrupted enamel plates emerges early in life and remains throughout the life of the individual. Evidence from fossil _Geomys_, especially from specimens from early and late Pleistocene deposits, suggests that the final adult pattern appears later, ontogenetically, than in Recent specimens. Some of the fossil premolars in initial stages of wear have continuous and uninterrupted bands of enamel. _Geomys quinni_ of the late Pliocene and early Pleistocene has the interrupted pattern seen in late Pleistocene and Recent _Geomys_. Also, in late Pliocene and early Pleistocene species, the re-entrant folds diverge laterally and form "open" angles. In later taxa (middle Pleistocene to Recent) the folds are compressed and parallel-sided, and the "open" folds are found only in the early stages of wear.

The posterior enamel plate of P4 disappears in the final stages of wear as the interrupted enamel pattern is formed. In the late Pleistocene and Recent _Geomys_, the loss of the posterior plate occurs early in life, usually in the first phases of wear on the occlusal surface of the newly erupted tooth, but in fossils of _Geomys_ of corresponding ontogenetic age from the early and middle Pleistocene, the posterior plate is retained in some individuals until a later phase of wear, thereby delaying the appearance of the final pattern. Indeed, in five or fewer per cent of the individuals (see Paulson, 1961:138-139; and White and Downs, 1961:18) a vestige of enamel is retained throughout life or at least until late in adulthood. In _Geomys tobinensis_, for example, a thin, but transversely complete, plate of enamel occurs all the way down to the base of the loph (Paulson, _loc. cit._) and would persist throughout life. In _Geomys garbanii_, a vestige on the lingual side of the posterior surface of a fully adult specimen was noted by White and Downs (_loc. cit._). Vestiges of the posterior plate occur less frequently in living geomyids. Paulson (_loc. cit._) found a posterior plate in one of 75 specimens of _Geomys bursarius dutcheri_. A young (suture present between exoccipitals and supraoccipital) female of _Geomys pinetis austrinus_ (KU 23358) has a vestige of the posterior plate on the lingual side of the tooth as White and Downs (_loc. cit._) observed in a specimen of _Geomys garbanii_. The enamel, I suspect, tends to be thicker on the lingual than on the labial side of the loph and extends farther down the lingual surface in some individuals; therefore, wear on the occlusal surface erodes it down to the dentine more rapidly on the labial than on the lingual side. The tendency of enamel to be retained is a primitive feature.

A lower molar of _Geomys_ is a single elliptical column, and enamel is restricted to the posterior surface as in _Zygogeomys_, _Orthogeomys_, and _Pappogeomys_. Paulson (_loc. cit._) found a thin enamel plate on the anterior surfaces of the lower molars in about five per cent of the individuals of _Geomys tobinensis_ from the Cudahy local fauna (middle Pleistocene, deposits of the late Kansan glaciation). An anterior plate is unknown in other members of the tribe Geomyini, except in the primitive genus _Pliogeomys_ of the middle Pliocene. Occurrence of the plate in _Geomys tobinensis_ is an atavistic trait. Primitive dental patterns occur occasionally in geomyids, as pointed out above, but the frequency of occurrence in _G. tobinensis_ is higher than would be expected.

M1 and M2, like the lower molars, are elliptical in cross-section. Complete enamel plates on the anterior and posterior surfaces are separated by tracts of dentine on the sides of each tooth. M3 is usually suborbicular (sometimes subtriangular) in cross-section. The tooth is not especially elongated posteriorly and usually has no definite heel; therefore, it is not significantly longer than wide. Living species of _Geomys_ rarely have a well defined outer re-entrant fold on M3; less than 10 per cent of the individuals (and usually only one side in each individual in which it occurs) have it, although a shallow inconspicuous groove occurs more frequently. The biprismatic molar characteristic of the ancestral morphotype is less often found in _Geomys_ than in any other living member of the tribe Geomyini. The outer re-entrant fold and biprismatic pattern are more often present in the extinct species _Geomys garbanii_ of the Middle Pleistocene than in other species. Less than 24 per cent of the third upper molars in _Geomys garbanii_ lack a tract of the re-entrant fold and more than 38 per cent have a well developed outer fold (see White and Downs, 1961:13, 18). The bicolumnar pattern, although incomplete, would be clearly evident in those teeth having a well marked re-entrant fold; the pattern occurs less frequently in those teeth with no fold or only a slight one. M3 of geomyids is not usually recovered and, therefore, the occlusal pattern of M3 is unknown in most extinct kinds of _Geomys_. In Recent _Geomys_ the fold is more common in the eastern _pinetis_ species-group than in the western _bursarius_ species-group.

The masseteric ridge on the outer side of the mandible is well developed in all species of the genus. The position of the mental foramen relative to the anterior part of the ridge varies with individuals and according to species. The basitemporal fossa is always present, but is shallower in the late Pliocene and Pleistocene species than in Recent species. The angular process is short.

_Referred species._--The twelve species, five of which are extinct, are as follows:

_quinni_ species-group

*_Geomys quinni_ McGrew, 1944. Geol. Ser., Field Mus. Nat. Hist.,
9 (546):49, January 20. Type from Sand Draw local fauna (late
Pliocene), Brown County, Nebraska; also known from
Broadwater-Lisco local faunas (early Pleistocene), Morrill and
Garden counties, Nebraska, Deer Park local fauna (early
Pleistocene), Meade County, Kansas.

*_Geomys paenebursarius_ Strain, 1966. Bull. Texas Memorial Mus.,
10:36. Type from Hudspeth local fauna (early Pleistocene),
Hudspeth County, Texas.

*_Geomys tobinensis_ Hibbard, 1944. Bull. Geol. Soc. Amer.,
55:736. Type from Tobin local fauna (middle Pleistocene),
Russell County, Kansas; also known from Cudahy local fauna
(middle Pleistocene), Meade County, Kansas.

*_Geomys garbanii_ White and Downs, 1961. Contrib. Sci., Los
Angeles Co. Mus., 42:1-34, June 30. Type from Vallecito Creek
local fauna (middle Pleistocene), San Diego County, California.

*_Geomys bisulcatus_ Marsh, 1871. Amer. Jour. Sci., 3:121. Type
from Loup River fossil beds, near Camp Thomas, Nebraska
(probably late Pleistocene).

_bursarius_ species-group

*_Geomys parvidens_ Brown, 1908. Mem. Amer. Mus. Nat. Hist.,
9:194. (An extinct subspecies of _Geomys bursarius_ according to
White and Downs, 1961:6). Type from Conard Fissure local fauna
(late Pleistocene), northern Arkansas.

_Geomys bursarius_ (Shaw, 1800). Trans. Linn. Soc. London, 5:227.
Type from somewhere in Upper Mississippi Valley, North America.

_Geomys arenarius_ Merriam, 1895. N. Amer. Fauna, 8:139, January 31.
Type from El Paso, El Paso County, Texas.

_Geomys personatus_ True, 1889. Proc. U. S. Nat. Mus., 11:159,
January 5. Type from Padre Island, Cameron County, Texas.

_pinetis_ species-group

_Geomys pinetis_ Rafinesque, 1806. Amer. Monthly Mag., 2 (1):45,
November. Type locality restricted to Screven County, Georgia.

_Geomys colonus_ Bangs, 1898. Proc. Boston Soc. Nat. Hist., 28:178,
March. Type from Arnot Plantation, about 4 mi. W St. Marys,
Camden County, Georgia.

_Geomys cumberlandius_ Bangs, 1898. Proc. Boston Soc. Nat. Hist.,
28:180, March. Type from Stafford Place, Cumberland Island,
Camden County, Georgia.

_Geomys fontanelus_ Sherman, 1940. Jour. Mamm., 21:341, August 13.
Type from 7 mi. NW Savannah, Chatham County, Georgia.

Genus =Orthogeomys= Merriam

1895. _Orthogeomys_ Merriam, N. Amer. Fauna 8:172, January 31.

1895. _Heterogeomys_ Merriam, N. Amer. Fauna 8:179, January 31
(type, _Geomys hispidus_ Le Conte, 1862).

1895. _Macrogeomys_ Merriam, N. Amer. Fauna 8:185, January 31 (type,
_Geomys heterodus_ Peters, 1865).

_Type._--_Geomys scalops_ Thomas, 1894, from Tehuantepec, Oaxaca, México.

_Chronologic range._--Late Pleistocene Wisconsin deposits (San Josecito Cave local fauna, Nuevo León, México) to Recent.

_Description and discussion._--Species of this genus are of medium to large size. The skull is strongly dolichocephalic in most species; the posterior part of the skull is especially narrow. The angular processes are remarkably short, especially in relation to the length of the mandible. The nasals and rostrum are relatively broad and heavy. The pelage is coarse, and often hispid. In some species the hairs are so sparsely distributed that the body appears almost naked, and none has so dense a covering of hair as do other genera. The genus occurs entirely within the tropical life-zones, and most of the external features seem to be associated with adaptation to tropical conditions.

The upper incisor is unisulcate; the sulcus is usually near the inner border of the tooth, but in some species (subgenus _Orthogeomys_) it is more medial, and in a few individuals with an extremely wide groove the outer lip of the sulcus may actually reach the middle of the tooth. The groove is compressed or open. The premolar is a double column united at the mid-point. The two prisms are of approximately equal size, and the lateral re-entrant folds are so compressed that their sides are parallel. Enamel plates cover the anterior surface and border the re-entrant angles in both upper and lower premolars. As in other members of the tribe, the lower premolar has a fourth enamel plate on the posterior surface of the posterior lophid. In the upper premolar, the enamel plate is reduced to a narrow blade on the lingual side of the loph as in the living species of the genus _Zygogeomys_. In the subgenus _Orthogeomys_ the posterior plate is usually absent, and otherwise is narrow and near the lingual border of the tooth.

Each lower molar, in the final stage of wear, consists of a single elliptical column having an enamel plate only on the posterior surface. The first and second upper molars are single elliptical columns having one enamel plate on the anterior surface and another on the posterior surface. The plates are separated by a tract of dentine on each side of the tooth. The third upper molar is partly bilophodont, and the two lophs are separated by a deep outer re-entrant fold. In many of the species an inner re-entrant fold also is retained, but in the adult tooth it is less distinct than the outer. In all of the species the posterior loph is long and forms a conspicuous heel; consequently the crown is significantly longer then wide. Moreover, the posterior loph has an enamel plate on each side. The labial plate always borders the outer re-entrant fold, and in the subgenus _Orthogeomys_ is infrequently separated into two small plates.

The mandible is relatively long. Its masseteric ridge is well developed and massive. The basitemporal fossa is usually deep and well defined; it tends to be shallow in the subgenus _Orthogeomys_, and in young individuals is hardly more than a slight depression.

Key to the Subgenera of _Orthogeomys_

A Frontal wide and greatly inflated; no interorbital
constriction; enamel plate on posterior wall of P4 usually
absent, although sometimes having small plate, restricted
to lingual end of wall. Subgenus _Orthogeomys_ p. 529

A´ Frontal narrow and not greatly inflated; interorbital
region decidedly constricted; enamel plate on posterior wall
of P4 always present but short and restricted to lingual end
of wall.

B Anterior margin of mesopterygoid fossa even with plane of
posterior wall of M3; postorbital bar weakly developed;
anteroposterior occlusal length of M3 equal to, or less
than, combined length of M1 and M2.
Subgenus _Heterogeomys_ p. 530

B´ Anterior margin of mesopterygoid fossa decidedly behind
plane of posterior wall of M3; postorbital bar strongly
developed; anteroposterior occlusal length of M3 more
than combined length of M1 and M2.
Subgenus _Macrogeomys_ p. 531

Subgenus =Orthogeomys= Merriam

1895. _Orthogeomys_ Merriam, N. Amer. Fauna, 8:172, January 31.

_Type._--_Geomys scalops_ Thomas, 1894, from Tehuantepec, Oaxaca, México.

_Chronologic range._--Known only from the Recent.

_Description._--Skull elongated and narrow (many skulls of nearly uniform breadth throughout), being extreme in dolichocephalic specializations; mandibles long and narrow, rami not spreading laterally, being more nearly parallel-sided than in other subgenera; angular processes short; breadth across zygomata not significantly exceeding breadth across mastoid processes (in many skulls considerably less); interorbital area remarkably broad, lacking deep constriction; frontals between orbits greatly inflated laterally, postorbital prominence inconspicuous; mesopterygoid fossa extending to level of posterior margin of M3; I having sulcus broader than in other subgenera, mostly on inner half of anterior surface but sometimes overlapping mid-line; enamel plate lacking from posterior wall of P4, rarely retaining narrow vestige near lingual border of posterior loph; M3 having distinct heel, bicolumnar pattern with inner re-entrant fold usually minute, occlusal length less than in other subgenera, length less than combined lengths of M1-2; hair generally coarse, sometimes hispid, sparse, in lowland forms, so sparse as to impart appearance of nakedness.

_Referred species and subspecies._--Fourteen taxa:

_Orthogeomys grandis alleni_ Nelson and Goldman, 1930. Jour. Mamm.,
11:156, May 9. Type from near Acapulco, 2000 ft., Guerrero.

_Orthogeomys grandis annexus_ Nelson and Goldman, 1933. Proc. Biol.
Soc. Washington, 46:195, October 26. Type from Tuxtla
Gutierrez, 2600 ft., Chiapas.

_Orthogeomys grandis carbo_ Goodwin, 1956. Amer. Mus. Novit.,
1757:5, March 8. Type from Excurano, 2500 ft., Cerro de San
Pedro, 20 km. W Mixtequilla, Oaxaca.

_Orthogeomys grandis felipensis_ Nelson and Goldman, 1930. Jour.
Mamm., 11:157, May 9. Type from Cerro San Felipe, 10 mi. N
Oaxaca, Oaxaca.

_Orthogeomys grandis huixtlae_ Villa, 1944. Anal. Inst. Biol. Univ.
Nac. México, 15:319. Type from Finca Lubeca, 12 km. NE
Huixtla, 850 m., Chiapas.

_Orthogeomys grandis grandis_ (Thomas, 1893). Ann. Mag. Nat. Hist.,
ser. 6, 12:270, October. Type from Dueñas, Guatemala.

_Orthogeomys grandis latifrons_ Merriam, 1895. N. Amer. Fauna,
8:178, January 31. Type from Guatemala, exact locality
unknown.

_Orthogeomys grandis nelsoni_ Merriam, 1895. N. Amer. Fauna, 8:176,
January 31. Type from Mt. Zempoaltepec, 8000 ft., Oaxaca.

_Orthogeomys grandis pluto_ Lawrence, 1933. Proc. New England Zool.
Club, 13:66, May 8. Type from Cerro Cantoral, north of
Tegucigalpa, Honduras.

_Orthogeomys grandis scalops_ (Thomas, 1894). Ann. Mag. Nat. Hist.,
ser. 6, 13:437, May. Type from Tehuantepec, Oaxaca.

_Orthogeomys grandis soconuscensis_ Villa, 1949. Anal. Inst. Biol.
Univ. Nac. México, 19:267, April 8. Type from Finca
Experanza, 710 m., 45 km. (by road) NW Huixtla, Chiapas.

_Orthogeomys grandis guerrerensis_ Nelson and Goldman, 1930. Jour.
Mamm., 11:158, May 9. Type from El Limón, in valley of Río de
las Balsas approximately 20 mi. NW La Unión, Guerrero.

_Orthogeomys cuniculus_ Elliot, 1905. Proc. Biol. Soc. Washington,
18:234, December 9. Type from Zanatepec, Oaxaca.

_Orthogeomys pygacanthus_ Dickey, 1928. Proc. Biol. Soc. Washington,
41:9, February 1. Type from Cacaguatique, 3500 ft., Dept. San
Miguel, El Salvador.

Subgenus =Heterogeomys= Merriam

1895. _Heterogeomys_ Merriam, N. Amer. Fauna, 8:179, January 21.

_Type._--_Geomys hispidus_ Le Conte, 1852, from near Jalapa, Veracruz.

_Chronologic range._--Late Pleistocene, Wisconsin deposits (San Josecito Cave local fauna, Nuevo León) to the Recent.

_Description._--Skull dolichocephalic (less so than in the other subgenera); zygomata more widely spreading than in _Orthogeomys_; ramus and angular process short; interorbital area noticeably constricted; frontals between orbits neither exceptionally broad or inflated; mesopterygoid fossa extending to level of posterior margin of M3; I having sulcus on inner third of anterior surface usually narrower than in subgenus _Orthogeomys_; enamel plate on posterior wall of P4 restricted to lingual half of loph; M3 distinctly biprismatic, posterior loph usually circumscribed by shallow inner re-entrant fold and outer deep fold well developed in all members of genus; posterior loph forming conspicuous heel longer than in subgenus _Orthogeomys_; occlusal length equal to or slightly less than combined lengths of M1-2; hair coarse and hispid but never so sparse as to impart appearance of nakedness.

_Referred species and subspecies._--Eleven taxa:

*_Orthogeomys onerosus_ (Russell, 1960). Univ. Kansas Publ., Mus.
Nat. Hist., 9 (21):544, January 14. Type from San Josecito Cave
local fauna, Upper Pleistocene, Nuevo León.

_Orthogeomys hispidus cayoensis_ (Burt, 1937). Occ. Papers Mus.
Zool., Univ. Michigan, 365:1, December 16. Type from Mountain
Pine Ridge, 12 mi. S El Cayo, British Honduras.

_Orthogeomys hispidus chiapensis_ (Nelson and Goldman, 1929). Proc.
Bio. Soc. Washington, 42:151, March 30. Type from Tenejapa, 16
mi. NE San Cristobal, Chiapas.

_Orthogeomys hispidus concavas_ (Nelson and Goldman, 1929). Proc.
Biol. Soc. Washington, 42:148, March 30. Type from Pinal de
Amoles, Querétaro.

_Orthogeomys hispidus hispidus_ (Le Conte, 1852). Proc. Acad. Nat.
Sci. Philadelphia, 6:158. Type from near Jalapa, Veracruz.

_Orthogeomys hispidus latirostris_ (Hall and Alvarez, 1961). Anal.
Escuela Nac. Ciencias Biol., 10:121, December 20. Type from
Hacienda Tamiahua, Cabo Rojo, Veracruz.

_Orthogeomys hispidus negatus_ (Goodwin, 1953). Amer. Mus. Novit.,
1620:1, May 4. Type from Gomez Ferias, 1300 ft., about 45 mi. S
Ciudad Victoria, 10 km. W Pan American Highway, Tamaulipas.

_Orthogeomys hispidus tehuantepecus_ (Goldman, 1939). Jour.
Washington Acad. Sci., 29:174, April 15. Type from mountains 12
mi. NW Santo Domingo and about 60 mi. N Tehuantepec, 1600 ft.,
Oaxaca.

_Orthogeomys hispidus torridas_ (Merriam, 1895). N. Amer. Fauna,
8:183, January 31. Type from Chichicaxtle, Veracruz.

_Orthogeomys hispidus yucatanensis_ (Nelson and Goldman, 1929).
Proc. Biol. Soc. Washington, 42:150, March 30. Type from
Campeche, Campeche.

_Orthogeomys lanius_ (Elliot, 1905). Proc. Biol. Soc. Washington,
18:235, December 9. Type from Xuchil, Veracruz.

Subgenus =Macrogeomys= Merriam

1895. _Macrogeomys_ Merriam, N. Amer. Fauna, 8:185, January 31.

_Type._--_Geomys heterodus_ Peters, 1865, from Costa Rica, exact locality unknown.

_Chronologic range._--Known only from the Recent.

_Description._--Skull dolichocephalic in varying degree (overlapping subgenera _Orthogeomys_ and _Heterogeomys_ in this respect); mandibles elongated, not spreading far laterally; angular processes decidedly short; breadth across zygomata in no instance significantly exceeding mastoid breadth; interorbital area strongly constricted; frontals between orbits slightly inflated laterally (especially in forms having more strongly dolichocephalic skulls); postorbital prominence conspicuous; anterior margin of mesopterygoid fossa terminating well behind M3; I having narrow and deep sulcus entirely on inner third of anterior surface; enamel plate on posterior wall of P4 restricted to inner half of loph; M3 bilophodont (outer and inner re-entrant folds each circumscribing a loph), posterior loph remarkably elongated and forming pronounced heel, length of crown more than combined lengths of M1-2; hair wooly in some individuals, harsh in others but seldom hispid, never so sparse as in subgenus _Orthogeomys_; some species having white markings, especially on lumbar region and head.

_Referred species and subspecies._--Eleven taxa:

_Orthogeomys heterodus cartagoensis_ (Goodwin, 1943). Amer. Mus.
Novit., 1227:2, April 22. Type from Paso Ancho, Province
Cartago, Costa Rica.

_Orthogeomys heterodus dolichocephalus_ (Merriam, 1895). N. Amer.
Fauna, 8:189, January 31. Type from San José, Costa Rica.

_Orthogeomys heterodus heterodus_ (Peters, 1865). Monatsb. preuss.
Acad. Wiss., Berlin, 1865:177. Type from Costa Rica, exact
locality unknown.

_Orthogeomys cavator nigrescens_ (Goodwin, 1943). Amer. Mus. Novit.,
1227:3, April 22. Type from El Muneco (Río Navarro), 10 mi. S
Cartago, 4000 ft., Province Cartago, Costa Rica.

_Orthogeomys cavator pansa_ (Bangs, 1902). Bull. Mus. Comp. Zool.,
39:44, April. Type from Bogava (= Bugaba), 600 ft., Chiriquí,
Panamá.

_Orthogeomys dariensis_ (Goldman, 1912). Smithsonian Misc. Coll.,
60(2):8, September 20. Type from Cana, 2000 ft., mountains of
eastern Panamá.

_Orthogeomys underwoodi_ (Osgood, 1931). Field Mus. Nat. Hist.,
Publ. 295, Zool. Ser., 185:143, Aug. 3. Type from Alto de
Jabillo Pirris, between San Geronimo and Pozo Azul, western
Costa Rica.

_Orthogeomys cherriei carlosensis_ (Goodwin, 1943). Amer. Mus.
Novit., 1227:3, April 22. Type from Cataratos, San Carlos,
Alajuela, Costa Rica.

_Orthogeomys cherriei cherriei_ (J. A. Allen, 1893). Bull. Amer.
Mus. Nat. Hist., 5:337, December 16. Type from Santa Clara,
Costa Rica.

_Orthogeomys cherriei costaricensis_ (Merriam, 1895). N. Amer.
Fauna, 8:192, January 31. Type from Pacuare, Costa Rica.

_Orthogeomys matagalpae_ (J. A. Allen, 1910). Bull. Amer. Mus. Nat.
Hist., 28:97, April 30. Type from Peña Blanca, Matagalpa,
Nicaragua.

Genus =Pappogeomys= Merriam

1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31.

1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31.
Type: _Geomys merriami_ Thomas.

1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31.
Type: _Geomys gymnurus_ Merriam; Hooper, Jour. Mamm.,
27:397, November 25, 1946.

_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.

_Chronologic range._--Late Pliocene, from deposits of early Blancan age (Benson local fauna, Arizona) to the Recent. However in the Pleistocene, only late Pleistocene records are known, and _Pappogeomys_ has not been found in early (late Blancan) or middle (Irvingtonian) Pleistocene local faunas. Presumably the genus was restricted to México during the Pleistocene until post-Wisconsin time.

_Description and discussion._--The size ranges from as little as in the smaller kinds of _Thomomys_ to the maximum attained in the subfamily and matched elsewhere perhaps in only a few of the larger subspecies of _Orthogeomys grandis_. Depending on the species and subgenus, the form of the skull varies from generalized to specialized. The generalized skulls are short and not especially narrow; the zygomatic arches are spread laterally so far that the breadth across them exceeds the breadth across the mastoid processes. The most specialized skulls are platycephalic and the breadth across the mastoid processes equals or exceeds the breadth across the zygomatic arches (even so, the zygomatic arches are still relatively widespread). In correlation with the great breadth of the posterior part of the cranium, the rami of the mandibles diverge widely posteriolaterally and the angular processes are remarkably elongated. The rostrum is moderately broad in most species, but not nearly so broad and heavy as in _Orthogeomys_.

The single deep, median sulcus on the outer surface of the upper incisor is slightly displaced to the inner side of the tooth. The posterior surface of P4 lacks enamel (small vestige found on lingual end of posterior wall in only two adult individuals--UA 3260 and KU 100442, of the subgenus _Pappogeomys_); the other three plates are fully developed as usual. The p4 is provided with four fully developed enamel plates, in the pattern characteristic of the tribe Geomyini. In the p4 of the late Pliocene species (_P. bensoni_) the re-entrant angles are open (obtuse), a trait that is evidently primitive in the Geomyini.

All three lower molars are single, compressed, elliptical columns with enamel on only the posterior surfaces. M1 and M2 are also elliptical in cross-section and decidedly anteroposteriorly compressed, like the lower molars. Nevertheless, the enamel pattern is variable; enamel plates may be retained completely across both the anterior and posterior walls of M1 and M2 or only the anterior plate may be retained without reduction and the posterior plate may be reduced so that only a vestige is retained on the lingual fourth of the tooth or the posterior plate may be completely lost.

M3 tends to remain at least incompletely bilophodont by reason of retaining a permanent labial re-entrant fold in most species (with exceptions in _Pappogeomys bulleri_ and some old adults of _P. castanops_). Primitively the occlusal surface of M3 is subtriangular (subgenus _Pappogeomys_), but in the _castanops_ species-group of the advanced subgenus _Cratogeomys_, the posterior loph usually is reduced and the occlusal surface is quadriform or obcordate. Curiously, the trend towards reduction of the posterior loph is reversed in one subspecies (_P. merriami fulvescens_) and, the loph has elongated into a pronounced heel in some specimens, resembling the condition in _Orthogeomys_. The entire range of variation occurs in _P. m. fulvescens_. The subtriangular pattern is retained in the most specialized species of _Cratogeomys_ where that pattern is associated with extreme platycephaly in the _gymnurus_ species-group. In most species the posterior loph supports two lateral plates, the outer one always bordering the labial re-entrant fold. In _Pappogeomys bulleri_ and in the _castanops_ species-group, the outer re-entrant fold of M3 tends to be obsolete, and the tooth becomes quadriform or suborbiculate in some individuals and loses the bilophodont pattern that characterizes other species. The lingual enamel plate is displaced to the posterior surface of the tooth, and one or both plates may disappear with advancing age. Consequently, only the anterior enamel plate remains in some adults, and constitutes the maximum degree of reduction of enamel on M3 in the Geomyinae. In many adults of _Pappogeomys bulleri_, the enamel investment of the posterior loph is complete and the two lateral plates are connected, without interruption around the posterior apex of the tooth, evidently representing the retention of a primitive character of the ancestral lineage.

The m3 of _P. bensoni_ from the late Pliocene is distinguished by minute lateral inflections suggesting the primitive biprismatic pattern. Also the posterior enamel plates of m1 and m2 are remarkably long, extending around the ends of the tooth. The associated upper incisor was unisulcate as in the modern species, and the basitemporal fossa of the mandible is well developed and deep.

The lower jaw is stout and relatively short. The masseteric ridge is well developed and has an especially thick crest. The basitemporal fossa is deep. In most living species, the pelage is soft and dense, but in one species, _Pappogeomys fumosus_, the hairs are coarse and hispid somewhat as in _Orthogeomys_.

Key to the Subgenera of _Pappogeomys_

A Enamel plates completely developed across posterior walls of
M1 and M2, except in one species (_P. alcorni_) having enamel
restricted to lingual fourth in M1; sagittal crest lacking
owing to impressions of temporal muscles remaining separated
(even in old adults); zygomata slender, and without platelike
expansion at lateral angle. Subgenus _Pappogeomys_ p. 534

A´ Enamel lacking on posterior walls of M1 and M2; pronounced
sagittal crest developed in adults of both sexes by union
of temporal impressions at middorsal line; zygomata stout
and wide, with lateral angle expanded into broad plate.
Subgenus _Cratogeomys_ p. 535

Subgenus =Pappogeomys= Merriam

1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31.

_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.

_Chronologic range._--Late Pliocene (Benson local fauna, Arizona) to Recent, but no specimens known from Pleistocene.

_Description._--Small, approximately same size as small subspecies of _Thomomys umbrinus_ but forefeet larger and claws longer; skull of generalized shape, broad, relatively short, smoothly rounded, not especially compressed dorso-ventrally; zygomatic breadth great but not exceeding mastoid breadth; zygomata relatively slender for geomyid and lacking platelike expansions at lateral angles; rostrum relatively narrow; sagittal crest lacking, owing to impressions of temporal muscles remaining separated; angular process of mandible not especially elongated; enamel plates extending completely across posterior wall of M1 and M2, except in one species, _P. alcorni_, where posterior plate of M1 remains only on lingual fourth of posterior wall (remainder of plate lacking); with wear, plates sometimes exceptionally thin completely across posterior face of M2 and especially M1 in a few individuals of _P. bulleri_ much as Paulson (1961:138-139) describes in extinct _Geomys tobinensis_; one or both plates rarely disappear in final stages of attrition in old individuals resulting in same dental pattern found in _Cratogeomys_; M1 and M2 retaining enamel plate on anterior wall throughout life; M3 usually subtriangular in cross-section but sometimes suborbiculate or ovoid, crown slightly bilophodont owing to shallowness of labial re-entrant angle in modern species; posterior loph of M3 not especially elongated and crown not significantly longer than wide; both lateral enamel plates of M3 usually well developed and approximately equal in length, occasionally plates reduced in length and rarely one or both plates are lost with wear in old individuals; patch of whitish or buffy hairs surrounding nose of most individuals.

The primitive character of the lower dentition, as described in the species account above, suggest that _Cratogeomys_ [= _Pappogeomys_] _bensoni_ Gidley should be referred to the subgenus _Pappogeomys_ rather than _Cratogeomys_. Only the upper dentition would make positive identification possible; however, reference to the subgenus _Pappogeomys_ seems to be the best arrangement at this time.

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

*_Pappogeomys bensoni_ (Gidley), 1922. U. S. Geol. Surv. Prof.
Papers, 131:123. Type from Benson local fauna (late Pliocene),
Cochise County, Arizona.

_Pappogeomys alcorni_ Russell, 1957. Univ. Kansas Publ. Mus. Nat.
Hist., 9(11):359. Type from 4 mi. W Mazamitla, Jalisco.

_Pappogeomys bulleri_ Thomas, 1892. Ann. Mag. Nat. Hist., Ser. 6,
vol. 10:196, August. Type from "near Talpa," west slope of
Sierra Madre de Mascota, Jalisco.

Subgenus =Cratogeomys= Merriam

1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31.

1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31.
Type: _Geomys gymnurus_ Merriam, 1892.

_Type._--_Geomys merriami_ Thomas, 1893, from "Southern México," probably in Valley of México.

_Chronologic range._--Late Pleistocene, from Wisconsin deposits (San Josecito Cave, Nuevo León, Upper Bercerra, México, and Burnet Cave, New Mexico, local faunas) to the Recent.

_Description._--Size medium to large; skull becoming angular and rugose with age, and tending towards platycephaly and dorso-ventral compression; zygomata stout, each bearing platelike expansion at anterolateral angle into which anterior end of jugal becomes morticed; breadth across zygomata great relative to length of skull; rostrum relatively broad; squamosals expanding medially with age eventually growing over lateral parts of parietals, and sometimes also expanding laterally displacing postglenoid notch; sagittal crest well developed in adults of both sexes, but especially high and bladelike in males; lambdoidal crest prominent in all but young animals, having dorsal outline broadly convex posteriorly in most species but strongly sinuous in _gymnurus_-group; enamel plate on posterior wall of P4 absent; enamel plates present only on anterior walls of M1 and M2; M3 variform in occlusal shape (as described in species account), either subtriangular (_gymnurus_-group), quadriform or obcordate (_castanops_-group, with exceptions as noted before); lateral plates of M3 usually present in all species, labial plate approximately as long as lingual plate in _gymnurus_-group (like that in subgenus _Pappogeomys_) or distinctly shorter in _castanops_-group (labial plate scarcely extending beyond border of labial re-entrant fold); one or both lateral plates tending to disappear with wear in _castanops_-group, with lingual plate usually disappearing first; breadth across angular processes clearly more than breadth across zygomatic processes, especially in _gymnurus_-group.

_Remarks._--In the species of the _castanops_-group the skulls can be spoken of as generalized and the least platycephalic of the subgenus. Indeed, the species of the _castanops_-group are hardly more specialized in this respect than is the subgenus _Pappogeomys_. In these skulls the breadth across the squamosal processes is less than that across the zygomatic arches, although the two dimensions are almost equal in some examples of _P. merriami_ of the _castanops_-group (where squamosal breadth varies from 85 to 98% of zygomatic breadth). In the species having marked platycephalic skulls (_gymnurus_ species-group) the breadth across the squamosal processes equals or exceeds the breadth across the zygomatic arches (squamosal breadth rarely 97 to 99% of zygomatic breadth), except in _P. zinseri_ and _P. tylorhinus zodius_.

The variable character of the third upper molar as between species suggests that this tooth is presently undergoing active evolution. The structure of this tooth, although differing between taxa, is remarkably stable in other kinds of Geomyini. The most remarkable modification of M3 in _Cratogeomys_ is the obcordate pattern developed in _P. merriami_ of the _castanops_-group. The posterior loph and entire tooth is shortened somewhat resembling in shape that of _Thomomys_. Moreover, the posterior loph is twisted labially; consequently, its posterior surface now forms the labial border of the weakly defined posterior loph. Owing to the torsion, the lingual enamel plate has been rotated to the posterior surface of the tooth. Therefore, the tooth is provided with two transverse enamel plates, including the plate on the anterior wall of the tooth. The labial plate is greatly reduced, its total surface being restricted to the small labial inflection. The highly specialized obcordate M3 is not found in the most specialized platycephalic skulls characteristic of the _gymnurus_ species-group. Instead the _gymnurus_-group retains the primitive subtriangular pattern without significant modification.

_Referred species._--Seven:

_castanops_ species-group

_Pappogeomys castanops_ (Baird, 1852). Report Stanbury's Exp'd. to
Great Salt Lake, p. 313, June. Type from "Prairie road to Bent's
Fort," near present town of Las Animas, Colorado.

_Pappogeomys merriami_ (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6,
12:271, October. Type from "southern Mexico," probably Valley of
México (see Merriam, 1895:152).

_gymnurus_ species-group

_Pappogeomys fumosus_ (Merriam, 1892). Proc. Biol. Soc. Washington,
7:165, September 29. Type from 3 mi. W Colima, Colima.

_Pappogeomys gymnurus_ (Merriam, 1892). Proc. Biol. Soc. Washington,
7:166, September 29. Type from Zapotlan (Ciudad Guzman),
Jalisco.

_Pappogeomys neglectus_ (Merriam, 1902). Proc. Biol. Soc.
Washington, 15:68, March 22. Type from Cerro de la Calentura,
about 8 mi. NW Pinal de Amoles, Querétaro.

_Pappogeomys tylorhinus_ (Merriam, 1895). N. Amer. Fauna, 8:167,
January 31. Type from Tula, Hidalgo.

_Pappogeomys zinseri_ (Goldman, 1939). Jour. Mamm., 20:91, February
15. Type from Lagos, Jalisco.

PHYLOGENY OF THE GEOMYIDAE

The fossil record of the Geomyidae provides a sequence of morphotypes, each representing a stage in the phyletic development of the family. Most of the preserved specimens probably represent the stufenreihe rather than the ahnenreihe, as Simpson (1953:219-220) points out. Even so, the stufenreihe closely approximates the general trend of evolution, and the level of structural organization in the different stages of phyletic development may be ascertained. The actual ancestral series of most lineages probably will remain unknown, but hopefully some of the existing gaps will be filled by future discoveries. From the established record, several clearly defined lineages can be distinguished; in fact the sequence of origin, pattern of evolution, and specializations, of the principal lineages are reasonably well expressed.

Primitive Morphotype

In the earliest known geomyids from the Upper Oligocene and Lower Miocene, the premolars and molars are biprismatic and bilophodont. In rodents, this is itself a specialized pattern, and is thought to have evolved from a more primitive sextituberculate prototype by the union of individual cusps, and probably also cuspules, forming the two transverse enamel lophs. The primitive, common ancestor of the Geomyidae and Heteromyidae with sextituberculate teeth in the early Tertiary is unknown.

As soon as geomyids attained the early bilophodont stage of evolution, the basic morphological structure of the family was established. The family probably first became clearly distinguished from other Geomyoidea at this stage. In the early bilophodont stages of evolution, owing to the relatively deep valley between them, the two columns probably failed to unite in the normal cycle of wear, as they do in all later geomyids. _Griphomys_ described by Wilson (1940:93) from the late Eocene of California, has a bilophate pattern in which the anterior and posterior lophs are separated by a persistent transverse valley. The occlusal pattern of _Griphomys_ closely resembles a stage through which the ancestors of the early Miocene geomyids must have passed in their pre-Miocene evolution, as Wilson suggests (1949:115-116). Although he (1940:95; 1949:110-118) tentatively referred _Griphomys_ to the superfamily Geomyoidea and Simpson (1945:80) went so far as to refer it to the family Geomyidae, with a notation of _incertae sedis_, its exact relationship to the pocket gophers is uncertain. However, the structure of the molariform dentition of _Griphomys_ does not exclude it from the phyletic ancestry of the Geomyidae. In subsequent stages of evolution the anterior and posterior columns become united. Thereby part of the valley floor between the transverse prisms was progressively elevated, to the stage where attrition on the occlusal surface would unite the two columns. On the unworn enamel cap of living geomyids the two transverse enamel folds are separated by a shallow but well defined valley, briefly reflecting the ancient ancestral pattern.

Union of the lophs may have been either at the mid-points of the two columns or at the edge of their protomeres. [A protomere is the half of a tooth containing the protocone or protoconid--lingual side of upper tooth and labial side of lower tooth. The paramere is the opposite half of a given tooth--labial side of upper tooth and lingual side in lower tooth. See Miller and Gidley, 1918:434.] Union of the columns at the mid-points would have produced the figure-8 occlusal pattern (or H-pattern), which is characteristic of the early Miocene Geomyinae (_Dikkomys_). Union of the two columns at the protomeres would have produced the U-shaped pattern of the Entoptychinae, which also occurred in the early Miocene and were contemporary with the earliest Geomyinae. Since pre-Miocene geomyids are unknown, the actual phyletic development of the dentition is a matter of speculation. Probably the development of the two divergent lineages, one leading to the Entoptychinae and the other to the subfamily Geomyinae, occurred in the Oligocene (as depicted in Fig. 3). Of the two lineages, the subfamily Geomyinae, in my view, is the more primitive and less specialized. Support for this view is furnished by a reconstruction of the pattern of occlusal wear in _Dikkomys_ and _Pliosaccomys_, especially on the first and second molars.