Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae

Chapter II: Part 2

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Upper Pleistocene records of _Geomys_ also are common. Upper Pleistocene is here understood to include late Illinoian, Sangamon and Wisconsin deposits; all are considered to be of Rancholabrean provincial age (see Hibbard _et al._, 1965:512-515) and post-Irvingtonian. The presence of remains of _Bison_ and/or _Microtus pennsylvanicus_ are currently considered mammalian index fossils of Rancholabrean faunas. In the Illinoian, _Geomys_ extended its range to the Atlantic Coast in the southeastern United States. The eastern and western species-groups evidently were isolated throughout much of the late Pleistocene, and, therefore, evolved separately. Of the two, the eastern, or _pinetis_, species-group seems to have remained somewhat more generalized, and the western, or _bursarius_, species-group has become more specialized. The Rancholabrean _Geomys_ from deposits in the southeastern United States are referable (see Ray, 1963:325) to _Geomys pinetis_.

Marsh (1871:121) described _Geomys bisulcatus_ from the North Prong of the Loup River (near Camp Thomas), Nebraska. These beds are also termed the Loup Fork or Loup River fossil beds (see discussion on p. 485), and they lie along the upper reaches of the Middle Loup River in Thomas County (near Senea), Hooker County (near Mullen), and southeastern Cherry County (probably the North Prong beds northwest of Mullen). These beds were at first thought to be of Miocene age, but later were regarded as early Pliocene (see Schultz and Stout, 1948:562-566 for a historical account of expeditions to these fossil sites). Schultz and Tanner (1957:71-72) pointed out that the principal fossiliferous beds in the Middle Loup region are of middle to late Pleistocene age, with most of the fossils coming from the Crete sand and silt beds which are probably early Illinoian deposits, and, therefore, younger than the Hay Springs faunas. Some fossils may have come from the Sappa deposits dated by Schultz and Tanner (_loc. cit._) as mostly Yarmouthian deposits. _Geomys bisulcatus_, judging from the original description and Hibbard's discussion of the cotypes (1954:357), does not differ significantly from _Geomys bursarius_. However, _Geomys bisulcatus_ is tentatively retained as a valid species. Based on the evidence cited above it seems unlikely that _Geomys bisulcatus_ occurred in pre-Irvingtonian times as often suggested in the literature.

The genus _Geomys_ has been identified in several faunas of Illinoian age, all from the Great Plains. Stephens (1960:1961) reported the genus from the Doby Springs local fauna in Harper County, Oklahoma, and Starrett (1956:1188) reported it from the Berends local fauna in Beaver County, Oklahoma. Schultz (1965:249) assigned 21 isolated teeth, including six incisors, from Butler Springs local fauna (considered by him to be late Illinoian, following the glacial maximum) to _Geomys_ cf. _bursarius_. Hibbard and Taylor (1960:167) reported a baculum tentatively identified as that of _Geomys_ from the early Illinoian Butler Springs local fauna (including the Adams fauna) of Meade County, Kansas. Hibbard (1963:206) recorded the genus _Geomys_ from the Mt. Scott local fauna (late Illinoian deposits) of Meade County, Kansas; the specimens probably are referable to the living species _bursarius_. From McPherson County, Kansas, Hibbard (1952:7) reported the genus _Geomys_ from the Kentuck Assemblage, which he (1958:25) regarded as a composite of Illinoian and Sangamon species. Specific identification of the Illinoian pocket gophers is uncertain, primarily due to the fragmentary nature of the material. On the basis of dental characters alone most specimens could be referred to _G. bursarius_; however the taxonomic status of _G. bisulcatus_ is in doubt, and more complete material may indicate that the Illinoian gophers are specifically distinct from the living species. Consequently, most authors, including myself, have made no attempt to refer these specimens to species. Nevertheless, the Illinoian _Geomys_ from the Great Plains is more closely allied to the living species of _Geomys_ than it is to the earlier Irvingtonian species.

_Geomys bursarius_ has been collected from a number of Sangamon fossil sites on the Great Plains. Although specific identification of specimens of _Geomys_ from Illinoian faunas is uncertain, the Great Plains _Geomys_ from Sangamon and later deposits probably is referable to the living species as Hibbard and Taylor (1960:165) pointed out. They found no difference between _Geomys_ recovered from the Cragin Quarry local fauna (early Sangamon) of Meade County, Kansas, and the living species _Geomys bursarius_. Isolated teeth of the same species were collected from the Jinglebob local fauna of Meade County, Kansas (Hibbard, 1955b:206), a fauna of the late Sangamon. Hibbard (1943:240) also recorded the genus _Geomys_ (referable to _G. bursarius_) from the Rezabek local fauna of Lincoln County, Kansas. According to Schultz _et al._ (1951:6 and table 1) the genus _Geomys_ occurs in buried or "fossil" soils of Sangamon age, lying just above the Loveland Loess, in Nebraska. No specific localities were given by them, nor were any particular specimens mentioned. Dalquest reported _Geomys bursarius_ from two Sangamon faunas in northern Texas. The species is represented in the Ward Quarry local fauna of Cooke County, Texas (1962a:42), and the Good Creek local fauna of Foard County, Texas (1962b:575).

_Geomys bursarius_ has been reported from Wisconsin fossil deposits of the Great Plains and adjacent areas as follows: Jones local fauna, Meade County, Kansas (Hibbard and Taylor, 1960:64-66); Two Creeks Forest beds of the third interstadial soils formed between Cary and Mankato glaciations, late Wisconsin (Schultz _et al._, 1951:8 and table 1); Cita Canyon local fauna in the northern part of the Panhandle of Texas (Johnson and Savage, 1955:39); Howard Ranch local fauna of Hardeman County in northwestern Texas (Dalquest, 1965:70); Quitaque local fauna of Motley County, Texas (Dalquest, 1964:501); Clear Creek local fauna of Denton County in north-central Texas (Slaughter and Ritchie, 1963:120); Ben Franklin local fauna, of late Wisconsin beds along the North Sulphur River in Delta County, NE Texas (Slaughter and Hoover, 1963:137); Bulverde Cave (Hay, 1920:140; 1924:247) and Friesenhahn Cave (Tamsitt, 1957:321), both in Bexar County, south-central Texas; Alton, Illinois (Hay, 1923:338-339); Wisconsin drift of Illinois, without mention of specific locality (Bader and Techter, 1959:172); Wisconsin drift of southwestern Wisconsin and northeastern Iowa (Hay, _op. cit._:343); Wisconsin drift near Galena, Illinois, and mouth of Platte River in eastern Nebraska (Leidy, 1869:406).

Brown (1908:194) described _Geomys parvidens_ from the Conard Fissure, in northern Arkansas. Hibbard (1958:25) concluded that the Conard Fissure fauna represents a glacial stage, probably the Illinoian, and Hibbard _et al._ (1965:510-511) regarded the fauna as a composite including both Irvingtonian and Rancholabrean elements. White and Downs (1961:21) considered _G. parvidens_ to be a subspecies of _Geomys bursarius_.

The first Pleistocene occurrence of _Geomys_ in the southeastern United States is from the Reddick I deposits reported by Gut and Ray (1963:325), who found the remains of _Geomys pinetis_ among the fossils comprising the "rodent beds" of Marion County, Florida. Gut and Ray tentatively identified the beds as Illinoian, but Kurten (1965:219) regarded the Reddick I fauna as early Sangamon. Simpson (1928:2) reported _Geomys floridanus_ [= _pinetis_] from Saber-tooth Cave deposits of Citrus County, Florida. The Saber-tooth Cave (or Lecanto Cave) local fauna is considered by Kurten (_op. cit._:219) also to be a Sangamon deposit. _Geomys floridanus_ [= _pinetis_] was reported from the Seminole Field deposits by Simpson (1929:563); both Simpson and Kurten (_op. cit._:221) agreed that the Seminole Field fauna is mainly late Wisconsin, although sub-Recent fossils occur at the tops of the beds. Ray (1958:430) collected remains of _Geomys pinetis_ from the Melbourne Bone Bed of Brevard County, Florida. The Melbourne local fauna is considered to be from Wisconsin deposits by Kurten (_op. cit._:220). The eastern species of _Geomys_ were probably derived from Great Plains stock that reached the southeastern Coastal Plains in early Rancholabrean (Illinoian) time. Presently there is no contact between the eastern and western populations of the genus, and it is assumed that disjunction occurred as a result of Wisconsin glaciation. It is interesting to note that the genus _Thomomys_ occurred in this region at approximately the same time; both genera occur in Saber-tooth Cave deposits.

_Pappogeomys_

The genus _Pappogeomys_ is not known from Pleistocene deposits older than the Wisconsin glaciation, but a pre-Pleistocene occurrence in the Benson beds of Arizona (see discussion of the Pliocene above) shows that _Pappogeomys_ had been differentiated by late Pliocene time. The absence of _Pappogeomys_, beginning in the early Pleistocene and continuing well into the late Pleistocene, is attributed to the southern distribution of the genus, where its range probably was centered on the Central Plateau of México. The paucity of early and middle Pleistocene deposits from this critical region prevents any definite statements about phyletic development within the genus. All of the late Pleistocene records pertain to the subgenus _Cratogeomys_ (long in use as a generic name but in the present paper reduced to subgeneric rank in the genus _Pappogeomys_). Schultz and Howard (1935:280) found _Cratogeomys_ [= _Pappogeomys_] _castanops_ in Burnett Cave in the Guadalupe Mountains of south-central New Mexico. The Burnett deposits are probably late Wisconsin (see Schultz and Tanner, 1957:75, for discussion of the age of these deposits based on carbon-14 tests). These writers (_loc. cit._) also referred the mandible of a small pocket gopher to the genus _Pappogeomys_ [= subgenus _Pappogeomys_]. However, neither genera nor subgenera of the tribe Geomyini can be distinguished on the basis of their inferior dentitions. Judging from the distribution of the modern geomyines, it seems unlikely that the subgenus _Pappogeomys_ has occurred beyond its present range in the late Pleistocene; therefore the small mandible is most likely that of a young individual of _Pappogeomys castanops_. Russell (1960:543) referred specimens collected at San Josecito Cave in Nuevo León, México, to the group of small subspecies _Cratogeomys_ [= _Pappogeomys_] _castanops_. Also, Russell (_loc. cit._) identified a rostral fragment as of the genus _Cratogeomys_ [= subgenus _Cratogeomys_] although the fragment had a combination of features different than in any named species of the genus; he did not name the fragment as a new species, preferring to wait for additional material that could clarify its taxonomic relationships.

Hibbard (1955a:52-53) identified _Cratogeomys_ [= _Pappogeomys_] _tylorhinus_ from the Becerra Superior deposits in the valley of Tequixquic in the northern part of the state of México. The Wisconsin age of these beds suggests an earlier Pleistocene derivation of the _gymnurus_-group of species.

Several specimens of the subgenus _Cratogeomys_ have been reported from beds of latest Wisconsin (certainly after the glacial maximum) or post-Wisconsin age. Gilmore (1947:158) found fossil remains of _Cratogeomys_ [= _Pappogeomys_] _castanops_ commonly in Quaternary cave deposits on the mountain slopes in the vicinity of Cuatro Ciénegas, in central Coahuila. These deposits actually may be of post-Wisconsin origin (see discussion above). Alvarez (1964:8) obtained fragments of _Cratogeomys_ [= _Pappogeomys_] _tylorhinus_ from sub-Recent deposits of Capa III in the Cueva La Nopalera in southwestern Hidalgo, México. _Pappogeomys merriami_ lives in the area today. Mayer-Oakes (1959:373) reported remains of _Cratogeomys_ [= _Pappogeomys_] _merriami_ from levels eight and eleven of the excavations at El Risco II, in the northern part of Mexico City. The ages of these deposits are unknown to me, but they probably are no older than late Wisconsin with most of the beds dating from the post-Wisconsin.

_Orthogeomys_

This genus is not known from the Pleistocene, except for its occurrence in the San Josecito cave deposits of southwestern Nuevo León, México (Russell, 1960:544). Although _Orthogeomys_ does not occur in the immediate vicinity of the cave at the present time, the northern limits of its range is nearby in southern Tamaulipas. The _Orthogeomys_ from San Josecito Cave differs from living species, and has been named _Heterogeomys_ [= _Orthogeomys_] _onerosus_ Russell (_loc. cit._), and is evidently referable to the subgenus _Heterogeomys_. As mentioned before, the San Josecito Cave local fauna represents deposits of Wisconsin glaciation.

HISTORY OF CLASSIFICATION

The account of the Tucan or Indian mole by Hernandez (sometimes listed as Fernandez) in 1651 probably is the earliest published one of a geomyid (see Merriam, 1895:201; Coues, 1877:607-608). Linnaeus in 1758 did not mention geomyids. In 1772, Kerr described Hernandez's Tucan under the name _Sorex mexicana_ on the basis of Hernandez's account without having seen any specimens. Lichtenstein in 1827 applied the technical name _Ascomys mexicana_ to three specimens collected by Deppe from unknown localities on the tableland of México. Merriam (_loc. cit._) pointed out that the name _mexicanus_ of Lichtenstein in 1827 is a _nomen nudum_, and that it is preoccupied by _mexicanus_ used by Kerr in 1792. The latter can not be technically identified with any particular species of geomyid.

Bartram in 1791 wrote of the pocket gopher of Florida, without formally describing it. The first available technical name is _Mus bursarius_ of Shaw in 1800. Rafinesque in 1817 proposed the first generic names for the geomyids when he described _Geomys_ and _Diplostoma_. In 1839, Waterhouse referred the genus _Geomys_ to his family Arvicolidae, considered by him to be a subgroup of muroids. In 1841, he suggested that _Geomys_ was related to _Bathyergus_ and _Spalax_. Waterhouse in 1848 (p. 8) treated the pocket gophers as a subgroup of rodents under the group name Saccomyina, in which he included the genera _Heteromys_, _Saccomys_, _Perognathus_, and _Dipodomys_. Hence, Waterhouse was the first to recognize the relationship between the heteromyids and geomyids. In the next year Gervais erected the family Pseudostomidae for a group of specialized squirrels to include _Geomys_ and _Thomomys_ and the same genera (at least in part) of heteromyids that Waterhouse classified in the "family" Saccomyina.

In 1839 the name _Thomomys_ was proposed by Maximilian (Wied-Neuwied). All of the generic names previously proposed for pocket gophers were considered by subsequent authors to be synonyms of _Geomys_.

A third family name, Sciurospalacoides, was proposed by Brandt (1855:188) who referred _Geomys_ and _Thomomys_ to that family. He placed his new family phylogenetically between the family Sciuridae and the family Spalacoides (a group in which Brandt included the genera _Spalax_, _Sipheus_, and _Ellobius_). Brandt took exception to the classification of Waterhouse (1848), who united the geomyids and heteromyids in one family. Brandt placed the heteromyid genera in other groups: _Perognathus_ in the Muridae, and _Macrocolus_ [= _Dipodomys_] in the Macrolini, a subfamily of the family Dipodoides.

Modern classification of the pocket gophers begins with Baird in 1858. The important classifications are summarized in Table 1; a few that do not depart essentially from those listed have been omitted owing to limited space for the tabular arrangement, but are discussed in the following account.

Baird probably was strongly influenced by the arrangement proposed by Waterhouse in 1848, but was opposed to separating geomyids from heteromyids as was done by Brandt. Baird was convinced of the close relationship of the geomyids and heteromyids, and referred both groups to one family, the Saccomyidae, as Waterhouse had done earlier. In order to recognize the morphological specializations he used two subfamilies, Geomyinae and the Saccomyinae. In the 20 years that followed, some authors followed Brandt and others followed Baird.

Gill, in 1872 (p. 71), proposed a classification essentially like Baird's of 1858, but Gill raised Baird's subfamilies to the rank of family (see Table 1). In referring all pocket gophers to the Geomyidae, Gill used that name as a family term for the first time. Also he established the superfamily Saccomyoidea to include his two families, Geomyidae and Saccomyidae; therefore, the Saccomyoidea was equivalent to the group Saccomyina of Waterhouse (1848) and the Saccomyidae of Baird (1858). Coues (1877), in his classic monograph of the Geomyidae followed the arrangement proposed by Gill in treating the pocket gophers as a family. Alston in 1876 proposed another classification based on Baird (1858), with two subfamilies, the Geomyinae and the Heteromyinae, united together in the family Geomyidae; thus, he recognized that the genus _Saccomys_ Frédéric Cuvier, 1823, was a synonym of _Heteromys_ Desmarest, 1817, as had been pointed out by Gray (1868:201) and Peters (1874:356). Coues (1877:487-490) acknowledged the invalidity of the genus _Saccomys_, but refused to give up the name in supergeneric classification. Winge, first in 1887 and subsequently in 1924, classified the geomyids and heteromyids together in the family Saccomyidae as did Baird in 1858, and like Coues, Winge too ignored the synonymy of _Saccomys_ with _Heteromys_ and insisted on retaining the technical terms Saccomyidae and Saccomyini.

Up to the time of Merriam's classic revision of the Recent Geomyidae in 1895 all the known species of living pocket gophers were referred to two genera, _Geomys_ and _Thomomys_. Merriam described much new material, especially from México and Central America, and proposed seven new genera (see Table 1). His complete and detailed study of the dentitions and osteology of the skull remains today as the definitive work on this subject, and is the point where most studies of the Geomyidae must begin. His treatment of the Recent genera survived for 52 years without change until Hooper (1946:397) arranged _Platygeomys_ as a synonym of _Cratogeomys_. However, Merriam's genera have been recognized in all subsequent classifications except for the current review (see Table 1).

Cope described the first known fossil geomyids in 1878, and published an excellent review of the two genera, _Pleurolicus_ and _Entoptycus_, in 1884 (pp. 855-870, pl. 64, figs. 1-9). Both genera were recovered from the John Day Miocene deposits of Oregon. Cope did not propose a new systematic arrangement of these geomyids, but referred them to the family Saccomyidae and mentioned that the Saccomyidae was equivalent to the family Geomyidae of Alston. Winge, in 1887, followed Cope in referring _Pleurolicus_ and _Entoptycus_ to the Saccomyidae along with the living genera _Thomomys_ and _Geomys_. Miller and Gidley (1918), in their synopsis of the supergeneric groups of rodents, proposed a new subfamily, Entoptychinae, to include the divergent Miocene pocket gophers. Miller and Gidley also revived the old subfamily Geomyinae of Baird (1858), but restricted its application to the modern pocket gophers and their immediate ancestors. In 1936, A. E. Wood revised the taxa of the subfamily Entoptychinae, and described the first Miocene genus, _Dikkomys_, of the Geomyinae. He followed the supergeneric classification of Miller and Gidley (1918).

The recent classifications of Simpson (1945) and Wood (1955) have combined the classifications of Merriam (1895) and Wood (1936). Wood (1955) brought up to date the list of genera, including those that were described after the publication of Simpson's classification (1945). In Table 1, the list of genera is principally from Simpson (1945) but generic names used by Wood (1955) are included. This is the currently accepted classification.

The new classification proposed in this paper (see Table 1) includes three tribes proposed as vertical units; they are intended to stress the phyletic trends in the known evolutionary sequences by placing immediate ancestors together with their descendants.

_Pliogeomys_ is placed in the same tribe (Geomyini) as _Zygogeomys_, _Geomys_, _Orthogeomys_, and _Pappogeomys_. That tribe includes the most specialized Geomyinae. _Zygogeomys_, _Geomys_, _Orthogeomys_, and _Pappogeomys_ are lineages resulting from a Pleistocene radiation in which all the lineages diverged from a common Pliocene ancestor. The radiation of the Geomyini was well under way by the close of the late Pliocene. Although _Pliogeomys_ may not be the actual ancestor, it closely resembles the primitive morphotype.

TABLE 1.--History of the classification of the Superfamily Geomyoidea

=====================+=====================+===================
Wood 1935 | Simpson 1945 | Names used in
Wood 1936 | Wood 1955 | present paper
---------------------+---------------------+-------------------
Family | Family | Family
Geomyidae | Geomyidae | Geomyidae
---------------------+---------------------+-------------------
Subfamily | Subfamily | Subfamily
Geomyinae | Geomyinae | Geomyinae
-- -- -- -- -- -- -- +-- -- -- -- -- -- -- +-- -- -- -- -- -- -
| | Tribe
| | Dikkomyini
| |
*_Dikkomys_ | *_Dikkomys_ | *_Dikkomys_
| *_Pliosaccomys_ | *_Pliosaccomys_
| |
| | Tribe
| | Thomomyini
| |
*_Pleisothomomys_ | *_Pleisothomomys_ | }
_Thomomys_ | _Thomomys_ | } _Thomomys_
| |
| | Tribe
| | Geomyini
| |
| *_Pliogeomys_ | *_Pliogeomys_
_Zygogeomys_ | _Zygogeomys_ | }
| *_Nerterogeomys_ | } _Zygogeomys_
| |
_Geomys_ | _Geomys_ | }
| *_Parageomys_ | } _Geomys_
_Orthogeomys_ | _Orthogeomys_ | }
_Heterogeomys_ | _Heterogeomys_ | } _Orthogeomys_
_Macrogeomys_ | _Macrogeomys_ | }
| |
_Pappogeomys_ | _Pappogeomys_ | }
_Cratogeomys_ | _Cratogeomys_ | } _Pappogeomys_
_Platygeomys_ | _Platygeomys_ | }
-- -- -- -- -- -- -- +-- -- -- -- -- -- -- +-- -- -- -- -- -- -
Subfamily | Subfamily | Subfamily
Entoptychinae | Entoptychinae | Entoptychinae
| |
*_Pleurolicus_ | *_Pleurolicus_ | *_Pleurolicus_
*_Gregorymys_ | *_Gregorymys_ | *_Gregorymys_
*_Grangerimus_ | *_Grangerimus_ | *_Grangerimus_
*_Entoptychus_ | *_Entoptychus_ | *_Entoptychus_
---------------------+---------------------+-------------------
| Geomyidae | Geomyidae
| _incertae sedis_ | _incertae sedis_
| |
| |
*_Gidleumys_ | *_Diplolophus_ | *_Diplolophus_
| *_Griphomys_ | *_Griphomys_
---------------------+---------------------+-------------------
Family | Family | Family
Heteromyidae | Heteromyidae | Heteromyidae
---------------------+---------------------+-------------------

* Denotes extinct genera.

** Winge included in his family Saccomyidae the "group"
Gymnoptychine and the contained genus _Gymnoptychus_ Cope, 1873,
which genus currently is placed in the family Eomyidae. The type
of _Gymnoptychus_ Cope, 1873, is synonymous with _Ischyromys_
Leidy, 1856, and the valid name for the genus is _Adjidaumo_
Hay, 1899.

_Pliosaccomys_, on the other hand, represents the terminal stages of a long trend that began with the _Dikkomys_-like Geomyinae of the early Miocene. In this lineage, the rate of evolution in the dentition and the skull was slow; therefore, the differences between early Miocene (_Dikkomys_) and middle Pliocene (_Pliosaccomys_) are not great and the two are united into the tribe Dikkomyini. The Dikkomyini is the ancestral geomyinen trunk from which the modern groups have diverged.

The Pliocene ancestor of _Thomomys_ is unknown but probably resembled _Pliosaccomys_, with which it may have been a contemporary. _Thomomys_ is the least specialized of the modern Geomyinae, and, consequently, shows the most resemblance to the ancestral tribe. The specializations of _Thomomys_, however, clearly preclude its reference to the tribe Dikkomyini; therefore, it is set apart in the monotypic tribe Thomomyini. That tribe has not undergone an adaptive radiation comparable to that of the tribe Geomyini or that of the Entoptychinae in the early Miocene. Here, for the first time, _Thomomys_ is set apart in classification from the other living pocket gophers.

Merriam's genera _Orthogeomys_, _Heterogeomys_, and _Macrogeomys_ are closely related. Each of these taxa is retained as a subgenus of a single genus, _Orthogeomys_. Some species of _Macrogeomys_ seem to be more closely allied to the subgenus _Orthogeomys_ and others to the subgenus _Heterogeomys_. A revision of the genus is needed; it might show that the currently recognized subgenera are artificial, and that a different arrangement of the species would more clearly express their evolutionary relationships. The subgenus _Heterogeomys_ seems to be the most nearly uniform of the subgenera, and it is the least specialized. Radiation within the genus may have begun relatively recently, but the many special adaptations for tropical environments suggest that the genus has been in the Neotropical Zone a long time. Therefore, discovery of an early dichotomy from the common ancestral stock of the tribe would come as no surprise.

_Nerterogeomys_ Gazin here is arranged as a junior synonym of _Zygogeomys_. Both are less specialized than any of the other Geomyini, except _Pliogeomys_. The single living species (_Zygogeomys tricopus_) is obviously a relic. Its range is small. The two subspecies differ only in minor features. The living species does have a few unique characteristics, only to be expected in the surviving species of a long phyletic lineage. Some of these are specializations. Otherwise, _Zygogeomys_ and _Nerterogeomys_ are closely related and the latter is best placed as a synonym of the former. Both are admittedly closely related to _Geomys_. _Zygogeomys_ and _Geomys_ share several characters, particularly primitive ones; there is considerable parallelism, especially marked in Irvingtonian species of _Geomys_. Nevertheless, _Geomys_ is more specialized, particularly in the dentition, and it has developed some _Pappogeomys_-like specializations. _Zygogeomys_ has retained more of the primitive characters of the tribe. A strong case could be made for recognizing only one genus, _Geomys_, containing _Zygogeomys_ as one of two subgenera. Nevertheless, the characters separating _Zygogeomys_ and _Geomys_ are of considerable importance and I consider the two kinds to be distinct genera.

The species of _Geomys_, both living and extinct, form a distinct and well-marked group. The genus is less primitive in most respects than _Zygogeomys_ and _Orthogeomys_ and it is less specialized than _Pappogeomys_, excluding the ancestral stock (subgenus _Pappogeomys_). Some specimens of species of Irvingtonian age (_Geomys tobinensis_ and _Geomys garbanii_, especially the former) retain primitive enamel plates as does _Zygogeomys_; but this is true of only a small percentage of the individuals. Also the adult dental pattern developed somewhat later in ontogeny in these middle Pleistocene species of _Geomys_ than in either Recent or late Pliocene and early Pleistocene representatives (_Geomys paenebursarius_, _Geomys quinni_) of the genus. Whether these features represent a stage in the evolution of the late Pleistocene and Recent species or a terminal stage in members of a sterile and primitive branch of the main line of evolution of _Geomys_ is uncertain. At present I favor the latter explanation, and view _G. paenebursarius_ and _G. quinni_ as early progressive species that evolved dental specializations that were maintained in the main line of phylogeny.

Hibbard proposed the generic name _Parageomys_ (1944:55), but later regarded it as a subgenus of _Geomys_ (1956:182) that includes those species retaining continuous enamel bands until relatively late in ontogeny; no other differences have been noted. When the early phylogeny of _Geomys_ is better understood, _Parageomys_ may serve as a subgeneric taxon in which the primitive species of _Geomys_ can be grouped, but as of now _Parageomys_ is arranged as a synonym of _Geomys_.

_Pappogeomys_ and _Cratogeomys_ also form a natural group. Their close relationship is best reflected in formal taxonomy by including them in the same genus. Their dissimilarities are of the sort that separate a primitive ancestral lineage from a divergent and progressively more specialized assemblage. The fossil record is inadequate, and I can only speculate that _Cratogeomys_ diverged from primitive _Pappogeomys_-stock in the earlier Pleistocene, at least before the end of the Irvingtonian. _Cratogeomys_ probably originated on the Mexican Plateau and probably underwent its subsequent evolution there. The living species of the subgenus _Pappogeomys_ are evidently relics of the ancestral stock of the genus. Hooper (1946:397), I think correctly, considered _Platygeomys_ as congeneric with _Cratogeomys_, although the highest degree of specialization of the genus is attained in those species formerly classed in the genus _Platygeomys_. Even so, in my opinion, the differences are insufficient to warrant even subgeneric recognition.

CLASSIFICATION

Family GEOMYIDAE Gill, 1872

Rodents of the superfamily Geomyoidea specialized for completely fossorial life (early Pliocene to Recent); specialized earlier (late? Oligocene and early Miocene) for semi-fossorial habits; body thickset, fusiform without apparent neck (in modern geomyids); legs short; forelegs especially stout; eyes and ears small (pinna reduced to inconspicuous crest concealed beneath pelage); tail tactile, shorter than head and body; lips closing behind incisors; cheek pouches external, fur-lined; baculum rodlike, arched, having expanded quadriform platelike base; pelage long, soft without underfur, covering body in thick coat (in some species of _Orthogeomys_ scant, harsh or scattered bristles); color varying from pale tints of buffy (almost white) to metallic black.

Skull thick-walled, massive, angular, relatively broad, and flattened; distinctly murine form, but having zygomasseteric structure of advanced sciuromorphs, including small infraorbital canal (that transmits no part of masseter muscle) and well-developed, broad zygomatic plate; zygomata massive and widely flaring, jugals stout; rostrum robust, relatively broad and deep, and without evidence of transverse canal (as in Heteromyidae); anterior projection of nasals only slightly exceeding that of upper incisors; interorbital region usually constricted, narrower than rostrum; anterior opening of infraorbital canal far forward on side of rostrum, about half way between zygomatic plate and upper incisor and just behind premaxillary-maxillary suture, its opening countersunk in oblique sulcus (for protection from muscle contraction); postorbital process lacking, except for rudimentary knoblike projection in subgenus _Macrogeomys_; palate relatively narrow, its deeply sculptured surface sloping steeply downward posteriorly causing region supporting maxillary tooth-row to be markedly depressed; palatine bone reduced, forming, on two abruptly different levels, posterior margin of hard palate behind tooth-rows; parietals compressed and narrow, and most of cerebral cavity roofed by squamosals (in some species squamosals overlap lateral parts of parietals); tympanic bullae completely inferior in position and fully ossified, external meatus being developed laterally as elongated tube; mastoid not inflated, but broadly exposed at posterolateral margin of the skull; occiput large, its surface usually rugose, and paroccipital processes large and flangelike, at least in advanced groups (early Pliocene to Recent); ramus relatively short and stout, having distinct crest and ridges for muscle attachments; coronoid process well developed, erect; articular condyle prominent; angular process prominent, reflected laterally, and in modern groups lateral extension protruding from posterior border of ramus nearly at right angle; capsule for root of lower incisor, prominent between angular process and articular condyle.

Anterior surface of incisors broad and flat, always smooth on lower teeth, but either smooth or grooved on upper teeth depending on taxon; cheek teeth hypsodont, becoming progressively higher crowned in modern groups, rooted in primitive groups (late? Oligocene to middle Pliocene), rootless and ever-growing in modern groups (late Pliocene to Recent); upper and lower premolars persistently bicolumnar; upper and lower molars bicolumnar only in primitive groups (late? Oligocene and early Miocene), becoming progressively monocolumnar in advanced groups (early Pliocene to Recent), primitive bicolumnar pattern being retained on occlusal surface only in early stages of ontogeny and in third molar throughout life; enamel pattern of occlusal surface of cheek teeth based on sextituberculate prototype (see Wood and Wilson, 1936:388-391), having cusps arranged in two transverse rows of three cusps each, excepting three anterior cusps of premolars that are arranged in trefoil, especially on p4 (sometimes only one or two, rather than three, cusps develop in a particular set, especially in p4), conules absent; protostyle and endostyle in upper teeth and protostylid and hypostylid in lower teeth formed from cingulum; cusps of each row uniting with wear into transverse enamel lophs (or lophids), each tooth having two lophs, one on anterior column, protoloph and protolophid, and one on posterior column, hypoloph and hypolophid, that unite with additional wear forming continuous enamel band; enamel lacking on sides of each column in advanced lineages, thereby restricting enamel to anterior and posterior walls; with extreme reduction, posterior plates of upper teeth and, more commonly, anterior plates of lower molars, missing. Dental formula: 1/1, 0/0, 1/1, 3/3.

Key to the Subfamilies of Geomyidae

A Angular process of ramus mostly below alveolar level of
mandibular tooth-row; pattern of premolar like that of molars,
consisting of two subequal crests united at one or both margins
of tooth; molars persistently bicolumnar; molariform teeth
always rooted. Subfamily Entoptychinae p. 513

A´ Angular process of ramus mostly above level of mandibular
tooth-row; pattern of permolar unlike that of molars,
consisting of two prisms differing in size and united at their
mid-points but never at either margin; molars progressively
monocolumnar, except for early Miocene forms; molariform teeth
rooted only in primitive genera (late? Oligocene to middle
Pliocene), and rootless and ever-growing in later genera (late
Pliocene to Recent). Subfamily Geomyinae p. 514

Subfamily ENTOPTYCHINAE Miller and Gidley, 1918

Anterior face of upper incisor usually smooth, sometimes bearing faint groove in center or near medial margin of tooth, at least in _Gregorymys_; cheek teeth hypsodont, medium to high crowned, and rooted in all but _Entoptychus_ (has rootless, ever-growing teeth); cheek teeth identical in form, premolars resembling molars and lower cheek teeth mirror images of upper teeth; crowns biprismatic, having two columns joined at edge of protomeres (for description of term, see discussion of primitive morphotype on page 537) and with persistent lateral fissure between them; lateral re-entrant fold deep, penetrating at least half width of crown, from external side in upper teeth and internal side in lower teeth (in specialized genus _Entoptychus_ lophs, upon additional wear, join also at edge of parameres, thus uniting columns at both ends and thereby enclosing interior part of lateral fissure as a transverse fossette in center of tooth); enamel investment of prisms usually complete, including inflection bordering re-entrant folds, occlusal pattern becoming interrupted with wear only in _Entoptychus_, where enamel disappears first from sides of crowns (following union of anterior and posterior columns at both sides) and later, in final stages of attrition, from anterior wall of lower molars and posterior wall of upper molars.

Maxillary bone without pronounced vertical depth in part supporting cheek teeth, its inferior border only slightly lower than inferior border of premaxillary and alveolar lips of molariform teeth consequently approximately level with, or slightly below, alveolar lip of upper incisor; squamosal without lateral expansion, therefore, meatal tube of auditory bulla separated from zygomatic process of squamosal by deep, well-developed postglenoid notch; angular part of mandible below alveolar level of mandibular cheek teeth; angular process only slightly reflected laterally; coronoid process low, tip only slightly above condyle.

For information concerning the structure and relationships of the known genera, and for accounts of species, see Wood (1936). A list of the named genera in order of specialization is as follows:

*_Pleurolicus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:66.

*_Gregorymys_ Wood, 1936. Amer. Mus. Novit., 866:9.

*_Grangerimus_ Wood, 1936. Amer. Mus. Novit., 866:13.

*_Entoptychus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:64.

Five new species have been described since Wood's (1936) revision. They are: _Pleurolicus clasoni_ MacDonald (1963:180); _Gregorymys kayi_ Wood (1950:335); _Gregorymys montanensis_ Hibbard and Keenmon (1950:198); _Grangerimus dakotensis_ MacDonald (1963:182); _Grangerimus sellardsi_ Hibbard and Wilson (1950:623).

Subfamily GEOMYINAE Baird, 1858

Anterior face of upper incisor primitively smooth, grooves consistently developed only in one modern lineage (Geomyini); cheek teeth hypsodont, primitively rooted and having crown of medium height (late Oligocene to middle Pliocene), being higher crowned, rootless and ever-growing in modern lineages (late Pliocene to Recent); primitively crowns of cheek teeth biprismatic, having two columns joined at mid-points by narrow isthmus and entire crown sheathed in continuous band of enamel; premolars retaining primitive biprismatic form, anterior and posterior columns never uniting at edge of protomeres or parameres, and with both lateral re-entrant folds persistent throughout life; primitive biprismatic pattern becoming decidedly modified in molars (except in M3), having two prisms progressively uniting into one column by reduction and loss of lateral inflections, primitive biprismatic patterns being retained only in early stages of ontogeny; third upper molars retaining, at least partially, primitive bicolumnar pattern (except in Thomomyini), with relatively broad isthmus and horizontally shallow re-entrant folds, lingual fold sometimes wanting; enamel pattern becoming discontinuous (late Pliocene to Recent) owing to loss of enamel from sides of each column; remaining enamel restricted to anterior and posterior plates, or cutting blades, and enamel bordering lateral inflections in premolars (considering both sides together, these plates constitute essentially two transverse cutting blades); enamel pattern of M3 varying, depending on taxon; with specialization, anterior plates of lower molars and posterior plates of upper premolar and molars may be reduced or lost; except in primitive species (early Miocene), no enamel fossettes retained in adult dentitions.

Maxillary bone having pronounced vertical depth in part supporting cheek teeth, inferior border arching downward well below inferior border of premaxillary; consequently, alveolar lips of molariform teeth decidedly below level of alveolar lip of upper incisor; squamosal with marked lateral expansion at expense of postglenoid notch; notch compressed and reduced between meatal tube of auditory bulla and zygomatic process of squamosal; angular part of mandible mostly above alveolar level of mandibular cheek teeth; angular process reflected laterally at right angles to axis of ramus and developed into heavy knoblike projection; coronoid process well developed, tip decidedly higher than condyle; fossorial specializations remarkably well developed in advanced lineages, degree of specialization of primitive Miocene species unknown but probably only semi-fossorial as in Entoptychinae.

Key to the Tribes of the Geomyinae

A Enamel investment complete and uninterrupted, even in final
(adult) stages of wear; cheek teeth rooted, with crowns of medium
height; third lower molar biprismatic, the two columns separated
by inner and outer re-entrant folds as in lower premolar.
Tribe Dikkomyini p. 515

A´ Enamel investment incomplete and discontinuous, reduced, at least
in final (adult) stages of wear, to interrupted enamel plates;
cheek teeth rootless and ever-growing (except in extinct genus
_Pliogeomys_), crowns of maximum height; third lower molar
monoprismatic, without trace of inner and outer re-entrant folds
as in first and second lower molars.

B Upper incisors smooth, occasionally with a fine indistinct
groove near inner margin of tooth; form of third upper molar
same as M1 and M2, monoprismatic, anteroposteriorly compressed,
and having transverse enamel plates on both anterior and
posterior faces, and without suggestion of either labial or
lingual re-entrant folds; basitemporal fossa absent (except
for a shallow depression in one Recent species, _T. townsendii_);
forefoot small and narrow with claws not elongated for digging.
Tribe Thomomyini p. 518

B´ Upper incisors grooved, bearing either one or two sulci; form of
third upper molar distinctly different from M1 and M2, fully or
partially biprismatic (with a few exceptions discussed beyond),
without marked anteroposterior compression (either subtriangular,
elongated, suborbicular or quadriform in cross-section, but not
elliptical as in M1 and M2), and having typical transverse
anterior plate and two lateral plates (varying in their
development, depending on taxa), but no posterior plate, and with
lateral re-entrant folds usually developed, especially labial
inflection (although sometimes minute in a few species, as
described beyond); basitemporal fossa well-developed, although
occasionally shallow or absent (primitive species of _Zygogeomys_);
forefoot large and broad, with elongated claws for digging.
Tribe Geomyini p. 521

Tribe DIKKOMYINI, new tribe

_Genotype._--_Dikkomys_ Wood, 1936.

_Chronologic and geographic range._--Early to Middle Pliocene (early Arikareean to mid-Hemphillian) in western United States. Known from Miocene fossil sites in Montana, South Dakota, and Nebraska and Pliocene sites in South Dakota, Oregon, Nevada, and southern California. For precise localities see accounts of _Dikkomys_ and _Pliosaccomys_ beyond.

_Diagnosis._--Small Geomyinae; lacking specializations of more advanced tribes; upper incisors smooth, at least in _Pliosaccomys_; molariform teeth always rooted and having crowns of medium height; enamel investment of cheek teeth complete and uninterrupted in all stages of wear; crowns of molars primitively biprismatic, having two columns united at mid-points, thus forming narrow isthmus separating lateral re-entrant folds as in premolars, and, with wear, also uniting secondarily at protomeres (with exception of third lower molars), consequently, isolating remnant of that inflection as shallow fossette (columns uniting first at protomeres in _Pliosaccomys_); anterior and posterior columns of first and second molars, both above and below, becoming progressively united into one column in advanced Dikkomyini (early and middle Pliocene), but m3 (M3 unknown) retaining primitive biprismatic pattern, with columns joined at centers but never at protomeres (for details of dentition see generic accounts); mandible stout, its angle mostly above mandibular tooth-row; masseteric ridge low; basitemporal fossa barely discernable in some fragments of _Pliosaccomys_; postcranial skeleton unknown.

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 _Thomomys_; known only from fragmentary mandible, including molariform dentition 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; masseteric 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 dentition), 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 _Thomomys monticola_; upper incisor relatively broad and flat, having anterior face smooth, without trace of grooving; crowns of cheek teeth of medium height and rooted; enamel investment continuous and uninterrupted in all stages of wear; premolars permanently, biprismatic; P4 having anterior prism subtriangular and decidedly smaller that sub-crescentic posterior prism, and joined near centers by narrow, obliquely oriented isthmus; p4 having anterior prism subovate, posterior prism strongly compressed anteroposteriorly, and joined at mid-points by relatively broad and straight isthmus; first and second molars, both above and below, monoprismatic in final (?adult) stage of wear, derived ontogenetically from primitive bilophate pattern by coalescence of two columns into one; M1 and M2 mirror images of m1 and m2 in pre-final stages of wear, two columns first uniting at edge of protomeres forming U-pattern, and primitive H-pattern never developing in either series (for detailed description of stages of wear, see account of phylogeny, p. 546); m3 (M3 unknown, but probably with same form as in Geomyini, see p. 552) persistently biprismatic, two columns joined by relatively broad isthmus at centers, consequently, forming H-pattern of primitive ancestors; rostrum heavy and broad as in modern geomyids; palate narrow and strongly ribbed; mandible stout; masseteric ridge and fossa well developed; basitemporal fossa absent.

_Specimens examined._--Six, nos. 1796 (holotype)--1799, 1804 and 1806 (CIT) now in the Los Angeles County Museum, all from Smiths Valley local fauna, Middle Pliocene, Nevada.

_Referred species._--two:

*_Pliosaccomys dubius_ Wilson, 1936. Carnegie Inst. Washington Publ.,
743:20, May 21. Known from early and middle Pliocene faunas
including Wolf Creek local fauna (late Clarendonian), Shannon
County, South Dakota; McKay Reservoir local fauna and Otis Basin
local fauna (Hemphillian), Oregon; type from Smiths Valley local
fauna (probably middle Hemphillian), Lyon County, Nevada.

*_Pliosaccomys wilsoni_ James, 1963. Univ. California Publ. Geol. Sci.,
45:101, June 26. Type from Nettle Springs local fauna of late
Clarendonian (early Pliocene), Ventura County, California.

Tribe THOMOMYINI, new tribe

_Type._--_Thomomys_ 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 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.

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Evolution and Classification of the Pocket Gophers of the Subfamily GeomyinaeChapter II: Part 2

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