Chapter I: Part 1

Transcriber's note:

Text enclosed by underscores is in italics (_italics_).

Text enclosed by equal signs is in bold face (=bold=).

Throughout, an asterisk (*) before a name denotes extinct
genera/species.

* * * * *

UNIVERSITY OF KANSAS PUBLICATIONS

MUSEUM OF NATURAL HISTORY

Vol. 16, No. 6, pp. 473-579, 9 figures in text

August 5, 1968

Evolution and Classification

of the Pocket Gophers of the

Subfamily Geomyinae

BY

ROBERT J. RUSSELL

UNIVERSITY OF KANSAS

LAWRENCE

1968

UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman, Henry S. Fitch, Frank B. Cross, J. Knox Jones, Jr.

Volume 16, No. 6, pp. 473-579, 9 figs.

Published August 5, 1968

UNIVERSITY OF KANSAS

Lawrence, Kansas

PRINTED BY

ROBERT R. (BOB) SANDERS, STATE PRINTER

TOPEKA, KANSAS

1968

31-4628

Evolution and Classification

of the Pocket Gophers of the

Subfamily Geomyinae

BY

ROBERT J. RUSSELL

CONTENTS

PAGE

INTRODUCTION 477

MATERIALS AND ACKNOWLEDGMENTS 477

TAXONOMIC CHARACTERS 478
Prismatic character of molars 478
Character of enamel patterns 479
Grooving of incisors 480
Masseteric ridge and fossa 480
Basitemporal fossa 481
Specializations of skull 481

FOSSIL RECORD 484
Miocene 485
Pliocene 486
Pleistocene 490
Thomomys 492
Zygogeomys 496
Geomys 496
Pappogeomys 503
Orthogeomys 504

HISTORY OF CLASSIFICATIONS 505

CLASSIFICATION 512
Family Geomyidae 512
Subfamily *Entoptychinae 513
Genus *_Pleurolicus_ 514
Genus *_Gregorymys_ 514
Genus *_Grangerimus_ 514
Genus *_Entoptychus_ 514
Subfamily Geomyinae 514
Tribe *Dikkomyini 515
Genus *_Dikkomys_ 516
Genus *_Pliosaccomys_ 517
Tribe Thomomyini 518
Genus _Thomomys_ 518
Subgenus *_Pleisothomomys_ 519
Subgenus _Thomomys_ 520
Tribe Geomyini 521
Genus *_Pliogeomys_ 522
Genus _Zygogeomys_ 523
Genus _Geomys_ 525
Genus _Orthogeomys_ 528
Subgenus _Orthogeomys_ 529
Subgenus _Heterogeomys_ 530
Subgenus _Macrogeomys_ 531
Genus Pappogeomys 532
Subgenus _Pappogeomys_ 534
Subgenus _Cratogeomys_ 535

PHYLOGENY OF THE GEOMYIDAE 536
Primitive Morphotype 537
Entoptychid Radiation 540
Phyletic Trends in Subfamily Geomyinae 542
Plio-Pleistocene Radiation of Geomyini 558
Morphotype 559
Specializations in Genera 560
Zygogeomys 564
Geomys 565
Orthogeomys 568
Pappogeomys 569

LITERATURE CITED 572

INTRODUCTION

When C. Hart Merriam wrote his monograph of the subfamily Geomyinae in 1895, he had no opportunity to examine fossil specimens. No doubt his phylogenetic conclusions and classification would have been greatly influenced had he enjoyed that opportunity because study of fossil geomyids reveals the historic sequence of phyletic development, and this sequence provides a firm basis for distinguishing specialized from primitive characters. The history of the Geomyinae has been characterized by the evolution of specializations. These evolutionary trends begin, as we presently know them, with a generalized ancestral stock in the early Miocene. The direction, degree, and rate of change, beginning with the primitive morphotype of the subfamily, has not been the same in the various lineages. The classification within the subfamily is based upon the phyletic interpretations of available data and the relationships they disclose. In turn, a new, and I hope more realistic, phylogeny and classification is offered.

MATERIALS AND ACKNOWLEDGMENTS

Recent specimens were studied of all the known genera, subgenera and 29 of the 36 living species. Most of the species not studied are monotypic and have restricted geographic ranges. They are: _Geomys colonus_, _G. fontanelus_, and _G. cumberlandius_, _Orthogeomys cuniculus_ and _O. pygacanthus_ of the subgenus _Orthogeomys_, and _O. dariensis_ and _O. matagalpae_ of the subgenus _Macrogeomys_. Examination of these modern species would not radically change the estimation of the degree of phyletic development of the genera and subgenera involved. All of the major polytypic and widespread species were studied.

Specimens of the extinct genera _Dikkomys_, _Pliosaccomys_, _Pliogeomys_, _Nerterogeomys_, and _Parageomys_ also were studied, as were examples of the extinct species _Geomys quinni_, _Geomys tobinensis_, and _Orthogeomys onerosus_. Considerable fossil material of living species, especially of the genera _Geomys_ and _Pappogeomys_, was used.

Inasmuch as the present account concerns mainly structural changes in the subfamily Geomyinae at the level of subgenera and above, and the temporal sequence of those changes, no attempt is made in the present account to revise taxonomy below the level of subgenera. Considerable modification of the classification below that level (for species and subspecies) is to be expected in _Orthogeomys_ and Pleistocene taxa of _Geomys_ when available specimens are studied.

I thank Prof. Robert W. Wilson for his assistance in securing fossil geomyids for study, and those in charge of the paleontological collections at the California Institute of Technology, Prof. Bryan Patterson, formerly of the Field Museum of Natural History, and Prof. Claude W. Hibbard of the University of Michigan, Museum of Zoology. For their kindness in lending Recent species, I thank Mr. Hobart M. Van Duesen of the American Museum of Natural History, Dr. David H. Johnson of the U. S. National Museum, and Dr. Oliver P. Pearson of the California Museum of Vertebrate Zoology, the late Colin C. Sanborn of the Field Museum of Natural History, and Profs. Emmet T. Hooper and William H. Burt of the University of Michigan Museum of Zoology.

I am especially grateful to Prof. E. Raymond Hall for his guidance and helpful criticisms with the manuscript. For assistance with paleontological problems, I thank Drs. Robert W. Wilson and William A. Clemens. Several persons have offered helpful suggestions and encouragement in the course of my study. For assistance of various sorts I especially thank Drs. J. Knox Jones, Jr., Rollin H. Baker, A. Byron Leonard, Sydney Anderson, James S. Findley, Robert L. Packard, and Robert G. Anderson. Advice concerning the drawings of the dentitions was generously given by Mr. Victor Hogg, and the drawings were done by Mrs. Lorna Cordonnier under his direction and by Mr. Thomas H. Swearingen. For assistance with secretarial tasks I thank Valerie Stallings, Violet Gourd, Ann Machin, Toni Ward, Sheila Miller, and my wife, Danna Russell.

TAXONOMIC CHARACTERS

Morphological features of the fossils and their stratigraphic provenience provide the information upon which phylogenetic interpretations are based. Although the most critical sequences of the fossil record are lacking, and although the existing fossils are mostly fragmentary and therefore seldom furnish ideally suitable data for the interpretations that have been made, phylogenetic conclusions drawn from fossil materials are superior to those drawn on other bases. The especially relevant characters are those disclosing primary trends in the evolution of the modern assemblages. The higher systematic categories recognized in the following account are based primarily upon such characters.

The most important characters found are in the teeth, although several structural changes in the lower jaw, especially those associated with the insertion of cranial musculature, are almost as important.

_Prismatic Character of Molars_

In primitive geomyines the molar consisted of two columns united at their mid-points and forming a figure 8 or H-pattern (see Fig. 4B). Both labial and lingual re-entrant folds were formed between the two columns. The primitive pattern is retained in the premolars of all known Geomyinae. Therefore, in the earliest (Miocene) members of the subfamily, the pattern of the molars was essentially like that of the premolars.

In Pliocene Geomyinae the two columns of the molars tend to merge into one. This is evident on the worn occlusal surface of the teeth; the lateral re-entrant folds are shallow vertically and progressively recede laterally until only a slight inflection remains. In the final stages of attrition, the inflection disappears and the tooth is a simple elliptical column. In the Pleistocene the monoprismatic pattern appears at earlier stages of wear owing to the decrease in depth of the re-entrant folds, and in Geomyinae of Recent time the initial stages of wear on the enamel cap of infants erase the last vestiges of two columns in the molar teeth.

The general trend in evolution, therefore, has been from a bicolumnar to a monocolumnar pattern. The particular patterns of wear characterizing each genus are described in detail beyond.

The third upper molar has evolved less rapidly than the first and second and in one of the modern lineages (tribe Geomyini) tends to retain at least a vestige of the primitive bicolumnar pattern in the final stage of wear. Therefore, the loss of any trace of the bicolumnar pattern in M3 is considered to be a much specialized condition. Unfortunately, the fossil record of the third upper molar is less complete than that for the first molar and second molar; the tooth drops out of its alveolus more often than does any one of the other molariform teeth and is seldom recovered.

_Character of Enamel Patterns_

In the primitive genera the enamel pattern is bilophate and the enamel loop (see Fig. 4B) is continuous on the occlusal surface of a worn molar. Concomitant with the union of the double columns, the bilophodont pattern is reduced to a single loph, but the enamel still completely encircles the dentine.

In the molars of modern geomyines, the enamel loop is not continuous but is interrupted on the sides of the crown by vertical tracts of dentine that are exposed at the occlusal surface of the tooth during early stages of wear. Therefore, a continuous enamel band is to be found only in a juvenal individual whose teeth have been subjected to only slight attrition on the enamel cap. In molars lacking enamel on the labial and lingual sides, anterior and posterior enamel plates, or blades, are found on each molar. The premolar also has an enamel plate on the anterior surface and another on the posterior surface, and in addition both re-entrant angles are protected by a V-shaped investment of enamel. One or the other of the various plates can be reduced or lost accounting for the several distinctive tooth-patterns of the modern geomyines. If loss occurs, it usually is the anterior plate in the lower dentition and the posterior plate in the upper dentition, including the upper premolar. When reduction of the posterior plate of the upper cheek teeth occurs, enamel is first lost from the labial side of the tooth, thus leaving only a short vestigial plate on the lingual end of the crown.

_Grooving of Incisors_

The incisors are smooth with no trace of a groove in the ancestral lineage. In the specialized assemblage (tribe Geomyini) pronounced grooves are always developed on the anterior face of the upper incisor. The pattern of grooving is constant in each species and thus provides characters of taxonomic worth for grouping species into genera. The only inconstancy noted was an incisor of _Geomys_ from the Tobin local fauna of the middle Pleistocene which has three grooves rather than the normal two (No. 6718 KU). The extra groove is an obvious abnormality, and the tooth was associated with others of the same species from the same quarry that were normally grooved.

Grooves on the lower incisors are unknown. The functional significance of grooving has been debated on numerous occasions in the literature. Grooves appear in a number of only distantly related rodents and in lagomorphs. The grooving occurs always in small herbivorous mammals, and in some way may be related to feeding habits.

The grooves provide a serrated cutting edge on the occlusal edge of the upper incisor. In the genus _Geomys_, for example, the two incisors, including the slight space between them, present a total of five serrations, which may facilitate cutting and piercing tuberous and fibrous roots upon which _Geomys_ feeds. Also the sulci would perform the same function as the longitudinal groove on the side of a bayonet, and would aid the animal in extracting its upper incisors from coarse, fibrous material. In gathering food, the gopher sinks its upper incisors into a root, and then, with the upper incisors firmly anchored, slices off small chunks by means of the lower incisors. Therefore, in pocket gophers, grooving may be an adaptation for feeding on fibrous or woody material. Finally, grooves increase the enamel surface of the incisor without additional broadening of the tooth itself. There could be a selective advantage for sulcation if the extra enamel and the serrate pattern strengthen the incisors, which are under heavy stress while penetrating or prying off pieces of coarse material. Few broken incisors of pocket gophers are found.

_Masseteric Ridge and Fossa_

This ridge and fossa are on the lateral surface of the ramus. The crest on the ridge begins at the base of the angular process and terminates slightly anterior to the plane of the lower premolar. The masseteric fossa receives the insertion of the rostral or superficial division of the masseter muscle. The mental foramen lies immediately anterior, or anteroventral, to the fossa.

In the ancestral lineage, the ridge is distinct but relatively low; the masseteric fossa is shallow and is a poorly developed area for attachment of the superficial masseter muscle. In modern Geomyinae the ridge is massive and forms a high crest, especially anteriorly, and the masseteric fossa is a deep, prominent cup along the dorsal side of the crest. The elaboration of the crest and fossa evidently is associated with an increase in size of the superficial masseter muscle, which enlarges and provides increased power for the propalinal type of mastication. A high crest has evolved independently in both modern lineages, Thomomyini and Geomyini.

_Basitemporal Fossa_

The name basitemporal fossa is suggested here to denote the deep pit that lies between the lingual base of the coronoid process and the third lower molar. The basitemporal fossa receives the insertion of the temporal muscle. The fossa, which until now has not been named, is a unique feature in advanced Geomyinae, being unknown in either primitive Geomyinae or in other rodents.

The temporal is one of several muscles holding the occlusal surface of the lower molariform dentition firmly against the upper cheek teeth during mastication. In primitive geomyines that masticate food by a planing action, the temporal muscle also moves the mandible posteriorly and food is ground between the enamel plates when the lower jaw is retracted as well as when it is moved forward.

The basitemporal fossa appears in late Pliocene geomyines and increases the attachment surface of the temporal muscles that powers the planing action important in utilizing woody and fibrous foods. The basitemporal fossa developed in only one of the modern lineages (tribe Geomyini), the same lineage in which grooved incisors evolved. Both features probably are adaptations for feeding on coarse food. The fossa is not greatly developed in either the ancestral tribe Dikkomyini or the modern tribe Thomomyini, although in some specimens a slight depression marks the site of the basitemporal fossa.

A. and B. Generalized type of skull. _Geomys bursarius lutescens_,
adult, male, No. 77955 KU, 10 mi. N Springview, Keya
Paha Co., Nebraska.
A. Dorsal view of skull.
B. Ventral view of lower jaw.
C. and D. Dolichocephalic type of skull. _Orthogeomys_ (_Orthogeomys_)
_grandis guerrerensis_, adult, female, No. 39807 KU,
1/2 mi. E La Mira, 300 ft., Michoacán, México.
C. Dorsal view of skull.
D. Ventral view of lower jaw.
E. and F. Platycephalic type of skull. _Pappogeomys_ (_Cratogeomys_)
_gymnurus tellus_, adult, female, No. 33454 KU, 3 mi. W
Tala, 4300 ft., Jalisco, México.
E. Dorsal view of skull.
F. Ventral view of lower jaw.
]

_Specializations of Skull_

The skull in most geomyines is generalized, being neither extremely long and narrow nor short, broad and flat as in specialized skulls (see Fig. 1). In Pleistocene lineages of the modern tribe Geomyini, long skulls and broad skulls evolved and have been termed dolichocephalic and platycephalic specializations, respectively by Merriam (1895:88-101). He correlated them with two diametrically different mechanical methods of mastication.

In animals with dolichocephalic skulls the principal movements of the mandible in the masticatory process are anteroposterior. The resulting propalinal action of enamel plates in opposition to each other characterizes also animals with a generalized skull, and evidently is the method of mastication in the primitive geomyines, but in animals with a dolichocephalic skull the method is developed to a high degree by elongation of the cranium, mandible, and teeth. Both the mandibular and maxillary tooth-rows are relatively longer than in the generalized skull, providing a longer block for the planing action of the lower molariform teeth. All teeth, especially P4 and M3, are longer. In M3 the heel (posterior loph) in particular is elongated. Both the anterior and posterior enamel plates usually are retained in M1 and M2.

The superficial (or rostral) masseter muscle, originates on the side of the rostrum and inserts in the masseteric fossa and on the masseteric ridge. The deep masseter, especially the zygomatic part having its origin along the zygomatic arch, inserts on the angular process of the lower jaw. These two divisions of the masseter muscle have a longer pull (forward) in the dolichocephalic skull than in a non-dolichocephalic skull. The temporal and diagastric muscles retract the lower jaws.

Other, secondary, modifications of the dolichocephalic skull are shortening of the angular process of the mandible, broadening of the rostrum, and narrowing of the cranium and zygomata. Depth of the posterior part of the skull is unchanged. The skull appears to be deep and of nearly equal breadth from nasals to occiput. A good example of a dolichocephalic skull is that of _Orthogeomys_ (see Fig. 1, C and D).

In the platycephalic skull, the principal masticatory movement of the mandible is anterooblique, to one side and then to the other. The oblique passage of the enamel blades of the lower teeth across those of the upper teeth produces a shearing rather than planing action (Fig. 1E, F). The anterooblique movement of the lower jaw is possible because of major architectural changes in the cranium and mandible. These changes include: (1) Broadening of the postrostral part of the skull, especially the occiput (mastoidal breadth equals or exceeds zygomatic breadth in skulls of some taxa); (2) flattening of the skull; (3) anteroposterior compression of the molariform teeth, especially the molars. Therefore, the entire maxillary tooth-row is relatively shorter than in the dolichocephalic skull. Only a vestige of the heel ordinarily remains on M3. The loss of the posterior enamel blades of P4, M1, and M2 eliminates unnecessary friction, and each of these teeth is wider than long. The distance between the posterior ends of the lower jaws is increased approximately in proportion to the extent that the occiput is widened. As a result of the flattening of the skull the angular processes of the lower jaws are lateral to the zygomatic arches, and approximately on the same vertical level with them. Consequently the insertions of masticatory muscles are shifted laterally. This is especially true of the zygomatic division of the deep masseter, which inserts on the angular process. Contraction of that muscle division of one side of the skull moves the lower jaws obliquely forward. The diagastric and temporal muscles of course retract the lower jaws.

The platycephalic skull is the most specialized skull in the Geomyinae and is a result of the new (for the Geomyinae) method of mastication. The subgenus _Cratogeomys_ (see Fig. 1, E and F) has a platycephalic skull. The trend toward platycephalic specialization has been the major feature of evolution in _Cratogeomys_.

FOSSIL RECORD

The fossil record of the subfamily Geomyinae begins in the early Miocene of western North America. No geomyids have been recovered from beds of the late Miocene age. Beginning with the early Pliocene the fossil record becomes progressively more complete, and geomyines are relatively abundant in deposits of late Pliocene and Pleistocene age. Although pocket gophers of the subfamily Geomyinae are rare in lower Miocene deposits, members of the subfamily Entoptychinae are relatively common and highly diversified. Four genera and a number of species have been described (see Wood, 1936:4-25), and the subfamily ranged widely in western North America. I interpret this to mean that the geomyines were indeed uncommon in the early Miocene and their distribution restricted since so few of their remains have been recovered in comparison with entoptychines and the known records are only from the northern part of the Great Plains. On the other hand, entoptychines enjoyed a widespread distribution in western North America (see discussion beyond). Probably the geographic range of the geomyines was largely allopatric to that of the more specialized entoptychines. The zone of fossoral adaptation for herbivorous rodents is ecologically narrow, and as a result competition is severe. As a rule, the outcome of episodes of intergroup competition is geographic exclusion. If these rodents were fossorial in the early Miocene--their morphology suggests they were at least semi-fossorial--mutually exclusive patterns of distribution are to be expected.

Miocene

_Dikkomys_ is the only genus of the Geomyinae known from the early and middle Miocene. _Dikkomys matthewi_ was described by Wood (1936) on the basis of isolated teeth from lower Harrison deposits (Arikareean in age) near Agate, Sioux County, Nebraska. Later, Galbreath (1948:316-317) described the features of an almost complete mandible recovered from the younger upper Rosebud deposits, now considered by MacDonald (1963:149-150) to be middle Miocene, near Wounded Knee, Shannon County, South Dakota. More recently Black (1961:13) has described a new species, _Dikkomys woodi_, from the Deep River Formation, Meagher County, Montana. The Deep River Formation is late Hemingfordian (middle Miocene) in age. No remains of _Dikkomys_ have been identified in the extensive rodent fauna of the John Day beds of the lower Miocene of Oregon, although entoptychines are abundant in these deposits.

In the present account, _Dikkomys_ is regarded as the ancestor from which the Pliocene and modern geomyines were derived. These probably did not evolve from the subfamily Entoptychinae because the dentition of entoptychines, especially the premolars and third molars, was already highly specialized by Miocene time.

The numerous records of _Thomomys_ and especially _Geomys_ reported from supposed Miocene or Pliocene deposits are without foundation (see Matthew, 1899:66; 1909:114, 116, 119; 1910:67, 72; 1923a:369; 1924:66; Matthew and Cook, 1909:382; Cook and Cook, 1933:49; and Simpson, 1945:80). Most of the records of _Geomys_ date back to the description of _Geomys bisculcatus_ Marsh (1871:121) from the Loup Fork beds of Nebraska (near Camp Thomas on the Middle Loup River). At first Marsh and other investigators thought these beds were of the late Miocene age. Subsequently the Loup Fork fauna was determined by Matthew (1923b) to be mostly early Pliocene (Clarendonian), but with a later Pleistocene element. Recently, Schultz and Stout (1948:560) have shown that the various Loup River faunas and also those from along the Niobrara River (Hay Springs, Rushville, Gordon local faunas) are of middle Pleistocene age, the fossil-bearing beds occurring just below the Pearlette Ash. These beds are those termed the Loup Fork or North Prong of Middle Loup by the earlier workers who supposed them to be of Miocene or Pliocene age. Both _Geomys_ and _Thomomys_ have been recovered from most of these deposits, but they are no older than middle Pleistocene. This is not surprising in view of the primitive structure of the geomyids known from Miocene and Pliocene beds, but the supposed early appearance of _Geomys_ and _Thomomys_ led to much confusion concerning geomyid evolution in the late Tertiary.

The dearth of geomyines in the Miocene is counterbalanced by the relatively abundant and highly differentiated gophers of the subfamily Entoptychinae. They reached the zenith of their development in this period. Four genera and a number of species are known from the western part of the United States, mostly from beds along the Pacific Coast and in the northern part of the Great Plains. The great diversification of the group in a relatively short period suggests prior movement into a new adaptive zone and subsequent specialization in different subzones and therefore an episode of radial adaptation. The radiation of the entoptychines is discussed elsewhere in the account of geomyid phylogeny, but it should be noted here that both the Geomyinae and the Entoptychinae appear in the fossil record at about the same time in the early Miocene. The principal distinguishing features of each of the two lineages were well developed at the time of their first occurrence, and the entoptychines were the more successful in early Miocene. The Entoptychinae are known only from the early and middle Miocene, unless the earlier deposits of the John Day Formation of Oregon from which mammals have been recovered are considered to be latest Whitneyian (latest Oligocene); for correlations, see Wilson (1949:75). Both lineages likely had an earlier history extending back to their divergence in the Oligocene.

Pliocene

The oldest and most primitive Pliocene geomyine is _Pliosaccomys dubius_ Wilson (1936:20) from the Smith Valley local fauna of middle Pliocene (Hemphillian) age in Nevada. According to Wilson (_op. cit._:15) the beds probably were deposited near the middle of Hemphillian time. Shotwell (1956:730) recorded _Pliosaccomys dubius_ from the McKay Reservoir and from the Otis Basin (1963:73) local faunas of the middle Pliocene (Hemphillian) of Oregon, and Green (1956:155) has recovered remains of _Pliosaccomys_ (cf. _dubius_) from the Wolf Creek local fauna, uppermost part of the lower Pliocene (late Clarendonian in age), of Shannon County, South Dakota. Recently, James (1963:101) has described a second species, _Pliosaccomys wilsoni_, of this primitive genus. The new species was found in early Pliocene deposits (late Clarendonian) from the Nettle Spring local fauna (Apache Canyon), in the Cuyama Valley, Ventura County, California. _Pliosaccomys wilsoni_ does not differ greatly from _P. dubius_; however, the few differences in dental characters seem to warrant specific recognition. The reduction of cusps on the metalophid of p4 from three (_dubius_) to two (_wilsoni_) and the lack of accessory cuspules on the protolophid of p4 in _wilsoni_ are probably specializations, suggesting that _P. dubius_ even though the more recent in age is the less advanced of the two. _P. wilsoni_ is known only from a lower jaw of a young individual that had dp4 in place, along with m1 and m2. The permanent premolar was in the process of erupting, and the deciduous tooth was removed so that the unworn surface of p4 could be examined.

_Pliosaccomys_ occurred geographically in the area that the Entoptychinae had occupied in the early Miocene. The Smith Valley material includes dentitions in almost all stages of wear and the chronological sequences in the development of the patterns of wear can be reconstructed. An understanding of the dental patterns of the primitive geomyines is based mostly on the interpretation of the stages of wear in _Pliosaccomys_.

No other pocket gopher is known from the area in which _Pliosaccomys_ occurred, and it is unknown after middle Hemphillian age. _Pliosaccomys_ has closer affinities with _Dikkomys_ of the early Miocene than with any geomyid of the modern assemblage and gives no clue to the origin of the lineage culminating in the modern pocket gophers of the tribe Geomyini.

_Pliogeomys buisi_ Hibbard (1954:353) was found in the Buis Ranch local fauna, of latest middle Pliocene, on the west side of Buckshot Arroyo, Beaver County, Oklahoma. The original material included a right ramus bearing the premolar and first two molars (the holotype) and five isolated premolars and molars. One of the molars is slightly worn and from an immature individual. One premolar is a deciduous tooth. Hibbard (_op. cit._:342) identified the beds from which he obtained the Buis Ranch local fauna as from the lowermost part of the Upper Pliocene. Moreover, he judged the Buis Ranch local fauna to be only slightly older than the Saw Rock Canyon local fauna of Seward County in southwestern Kansas. Previously (Hibbard, 1953:408-410), the Saw Rock Canyon local fauna had been assessed as older than the Rexroad local faunas (latest late Pliocene) and, therefore, representative of the early part of the late Pliocene. More recently, Hibbard (1956:164) identified the Buis Ranch beds as part of the Ogallala Formation, which here occurs unconformably just beneath the Rexroad Formation (composed of strata nearly all of late Pliocene age). Therefore, he regarded the Buis Ranch beds as latest middle Pliocene in age. Hibbard (1954:356) suggested that pocket gopher remains from the Saw Rock Canyon local fauna were referable to _Pliogeomys buisi_, and, in effect, tentatively assigned them to _Pliogeomys_ (in his description of the genus Hibbard remarked that the upper incisor is bisulcate as in _Geomys_, and the only upper incisor that he mentions was one of the Saw Rock Canyon fossils and not part of the Buis Ranch material). _Pliogeomys_ has closer affinities with modern pocket gophers of the tribe Geomyini than it does with the middle Pliocene genus _Pliosaccomys_.

The pocket gopher fauna known from the late Pliocene was more varied than the faunas known from any earlier time. In addition to the extinct _Pliogeomys_, which occurs in early late Pliocene (see discussion above), the living genera _Zygogeomys_, _Geomys_, _Pappogeomys_ (in the sense used on p. 534), and _Thomomys_ first appear in the late Pliocene. The only other living genus, _Orthogeomys_, makes its first appearance in the late Pleistocene.

The earliest record of the genus _Thomomys_ is based on a fragment of a left mandibular ramus bearing p4 and m1, _Thomomys gidleyi_ Wilson (1933b:122), from the Hagerman local fauna of Twin Falls County, Idaho. Wilson (_loc. cit._) was uncertain as to age (late Pliocene or early Pleistocene) but subsequently (1937:38 and 67-70) settled on the middle part of the late Pliocene. Hibbard (1958:11) later considered the age as early Pleistocene (suggesting that the deposits accumulated in the Aftonian interglacial interval) but subsequently (Hibbard _et al._, 1965:512), on the basis of potassium argon age determinations, also settled on late Pliocene.

Remains of _Nerterogeomys_ [=_Zygogeomys_] have been found in the Benson local fauna, Cochise County, Arizona, and the Rexroad local fauna of Kansas. This early Blancan gopher first was described as _Geomys minor_ by Gidley (1922:123), and was later referred by Gazin (1942:487) to his new genus _Nerterogeomys_. Hibbard (1950:138) identified specimens from the Fox Canyon locality, one of the localities of Meade County, Kansas, where the Rexroad local fauna is preserved, as _Nerterogeomys_, and tentatively referred them to the species _N. minor_. _Nerterogeomys_ cf. _minor_ has been recovered also from Locality 3 of the Rexroad local fauna (Hibbard, 1950:171) of Meade County, Kansas. Apparently these are also the small gophers about which Franzen (1947:58) wrote. She assigned them to the genus _Geomys_, and they may actually be a primitive form of _Geomys_ that represents an intermediate stage in the development of the enamel pattern from the uninterrupted loops of the ancestor to the discontinuous pattern of modern _Geomys_. I favor this interpretation; the evidence, however, is inconclusive, and I have, therefore, reluctantly allocated them, along with the other specimens of _Nerterogeomys_, to the genus _Zygogeomys_. In an early paper, Hibbard (1938:244) erroneously referred the same specimens, two upper premolars of a young individual, to the genus _Thomomys_, and the same material was identified with the genus _Geomys_, also without specific assignment, in a later paper (Hibbard, 1941b:278). _Thomomys_ is unknown from the late Pliocene of the Great Plains. The specimens previously referred to _Nerterogeomys_ are assigned to the genus _Zygogeomys_ for the first time in this report; for a discussion of the systematic arrangement see the accounts beyond. The type and paratype of _Nerterogeomys_ from the Benson local fauna of Arizona have no indication of enamel reduction.

Specimens of the genus _Geomys_ from the late Pliocene were referred to the large _Geomys quinni_ McGrew, first by Franzen (1947:55) and later by Hibbard and Riggs (1949:835) and Hibbard (1950:171). _Geomys quinni_ has been obtained from the Fox Canyon locality and Locality 3 of the Rexroad local fauna. At Locality 3, both _Zygogeomys_ (cf. _minor_) and _Geomys quinni_ have been found together, but _Geomys quinni_ can be distinguished by its much larger size and the advanced enamel pattern of the cheek teeth (see systematic accounts beyond). All age classes are represented among the specimens of _Geomys quinni_; therefore, it seems unlikely that the smaller gophers referred to _Zygogeomys_ are actually the young of _Geomys quinni_. Hibbard (personal communication, May, 1966) informed me that specimens of _Geomys_ from the late Pliocene (Fox Canyon and Rexroad Locality 3) are erroneously referred to _G. quinni_. According to Hibbard, this material represents instead two distinct undescribed species, descriptions of which have been submitted by him for publication. Allocation of late Pliocene specimens of _Geomys quinni_ to other species will restrict _quinni_ to the early Pleistocene.

_Cratogeomys bensoni_ Gidley (1922:123) was of medium size. The name was based on an upper incisor bearing a single median sulcus and an associated lower jaw containing all of the cheek teeth from the Benson local fauna, Cochise County, Arizona. Additional lower jaws carrying various teeth also were recovered. The specimens might just as well have been assigned to the genus _Pappogeomys_ since the lower dentitions of all the genera of the tribe Geomyini have the same enamel pattern, and the subgenera _Pappogeomys_ and _Cratogeomys_ have upper incisors with median grooves. The specimens are too fragmentary to warrant more than generic identification. Mainly because of their late Pliocene age and primitive traits the specimens are here regarded as early representatives of the subgenus _Pappogeomys_. Discovery of the upper molariform dentition would make a more precise assignment possible.

Pleistocene

Numerous specimens of geomyids from many localities and horizons are available from the Pleistocene of North America. Specimens of the genera _Geomys_ and _Thomomys_ are especially common. Few specimens are known of the genera _Orthogeomys_ and _Pappogeomys_, especially from the early and middle Pleistocene, owing, probably, to slight knowledge of the early Pleistocene of México where these two genera are thought to have evolved (see map, Figure 2). This lack of knowledge about early Pleistocene deposits in México is a handicap in the present instance since the center of differentiation for several of the modern genera is judged to have been in México, probably on, and at the edge of, the Central Plateau. The relative abundance of the remains of _Geomys_ and _Thomomys_ from Pleistocene deposits farther north, and the marked absence of other genera, may mean that _Orthogeomys_ and _Pappogeomys_ did not range northward from southern and central México in most of the Pleistocene. One species of _Pappogeomys_ eventually ranged into the southwestern United States in the late Pleistocene (toward the end of the Wisconsin) and it occurs there today, but the genus is essentially Mexican.

The fossil record of _Zygogeomys_, as the genus is here understood, evidently continued in the United States will into the Middle Pleistocene, depending upon the stratigraphic interpretation of the age of the Curtis Ranch local fauna from southeastern Arizona. Hibbard (1958:25) regarded the Curtis Ranch local fauna as Irvingtonian in age, a local fauna that lived either in the late Kansan glacial or the Yarmouthian interglacial, and his correlation is tentatively followed here. In deposits laid down later than those of Irvingtonian age no remains of _Zygogeomys_ have been found. Today a single species exists as a relic in the mountains of central México and _Zygogeomys_ may have retreated southward to its present refugium in the late Pleistocene. Perhaps, _Zygogeomys_ occurred in northern México and the southwestern United States in the early and middle Pleistocene (see Fig. 2), occupying the area between the ranges of _Pappogeomys_ to the south and _Geomys_ to the north. Competition with _Pappogeomys_, and especially _Geomys_, during Irvingtonian time may have extirpated _Zygogeomys_ over most of this area, and by late Pleistocene (Sangamon) much of the former range of _Zygogeomys_ came to be occupied by one or the other of its competitors. The occurrence of _Geomys garbanii_ in southern California (see White and Downs, 1961) and the unidentified species of _Geomys_ in Aguascalientes (Mooser, 1959; for faunal correlation, see Hibbard and Mooser, 1963), both from deposits of Irvingtonian age, supports this suggestion.

1. _Thomomys_
2. _Geomys_
3. _Zygogeomys_
4. _Pappogeomys_
5. _Orthogeomys_
]

_Thomomys_

The earliest Pleistocene records of _Thomomys_ are mostly isolated teeth. Although they can be identified as genus _Thomomys_, most of the materials are too fragmentary to be identified to species. In _Thomomys_ two distinct patterns of occlusal surfaces of the molars can be recognized: the generalized elliptical pattern in the subgenus _Pleisothomomys_, not unlike the pattern in other geomyids, and the pear-shaped pattern in the subgenus _Thomomys_, which results from constriction of the upper molars on the labial side and constriction of the lower molars on the lingual side. Some fossils assigned to _Thomomys_ were not examined with this distinction in mind by the persons who made the assignments. Consequently some of the identifications now in the literature may be subject to change.

Three occurrences of _Thomomys_ are from the early and middle Pleistocene, with a possible fourth (depending upon the age of the Hay Springs local fauna of Nebraska). The earliest Pleistocene record is from the Broadwater-Lisco beds along the North Platte River in Morrill County, western Nebraska. Possibly the specimen from there was misidentified. Those beds are Lower Pleistocene, and are regarded by Schultz and Stout (1948:560-561, 573) and by Hibbard (1958:11), as having been deposited mostly during the Aftonian interglacial. There is also some indication that some of the strata were deposited late in the Nebraskan glaciation. There are no other early Pleistocene records of _Thomomys_. Savage (1951:228) reported the genus from the Irvington local fauna, Alameda County, California. The specimens were not identified to species, although they were described as indistinguishable from _Thomomys bottae_. Paulson (1961:137) recorded specimens from the Cudahy local fauna, Meade County, Kansas. These fragmentary specimens are referable to the subgenus _Thomomys_, owing to the strong constriction of the molars, but have not been identified to species. The Cudahy is an Irvingtonian local fauna, and is considered to have been deposited during the late Kansan glaciation. The stratum containing the Cudahy local fauna immediately underlies the Pearlette Ash. The Cudahy material includes five isolated molars and a fragmentary ramus bearing only the premolar. The genus _Thomomys_ has been recovered also from the Hay Springs local fauna in Sheridan County, northwestern Nebraska, by Shultz and Tanner (1957:71). The Hay Springs local fauna is considered to have been deposited in late Kansan glaciation or in early Yarmouth interglacial by Shultz and Tanner (_op. cit._:69), or of Irvingtonian age; however, Hibbard (1958:25) regarded the beds containing this fauna as Illinoian (thus post-Irvingtonian in age), and equivalent in age to the Berends local fauna of Oklahoma and the Butler Springs and Mt. Scott local faunas of Kansas. The _Thomomys_ from Hay Springs local fauna has not been referred to species.

The relative abundance of _Geomys_, and rarity of _Thomomys_, in Great Plains fossil beds of early and middle Pleistocene is probably due to allopatric distributions of the two genera. The Great Plains area was evidently the center of distribution and differentiation of _Geomys_. Perhaps _Thomomys_ evolved earlier to the west, in the Great Basin and Pacific Coastal regions, and not on the Great Plains.

Upper Pleistocene records of _Thomomys_ are more common. The genus was widespread in beds identified with the Illinoian and Sangamon and extended its range eastward to the Atlantic Coast. Stephens (1960:1961) reported _Thomomys_ from the Doby Springs local fauna, Harper County, northwestern Oklahoma. The material (34 isolated teeth) was too fragmentary to permit assignment to species. The molars are constricted on one side, indicative of the subgenus _Thomomys_, like the Cudahy specimens reported by Paulson (see discussion above). Stephens erroneously mentioned that the enamel plate on the posterior face of the upper premolar is unique in _Thomomys_; this plate occurs also in _Zygogeomys_. The Doby Springs local fauna was recovered from beds that have been identified as Illinoian deposits, and it is correlated with the Berends local fauna in Beaver County, Oklahoma, and the Butler Springs local fauna in Meade County, Kansas (see Stephens, _op. cit._: 1700).

Local faunas in Maryland and Florida of Rancholabrean age include _Thomomys_, in every instance referable to the subgenus _Pleisothomomys_ on the basis of unconstricted molars. _Thomomys potomacensis_ (Gidley and Gazin, 1933), from Cumberland Cave local fauna, Allegany County in western Maryland, is the type of the genus _Pleisothomomys_ Gidley and Gazin (1933:354). _Pleisothomomys_ is here regarded as a subgenus. The material used in the original description included four lower jaws, one with a complete dentition. Hibbard (1958:25) pointed out that the Cumberland Cave assemblage is a composite fauna including both glacial and interglacial forms. He placed the stratigraphic position of the fauna as definitely Upper Pleistocene, probably deposited in both Illinoian glaciation and during the Sangamon interglacial. _T. potomacensis_ is significantly larger than _T. orientalis_ Simpson (1928:6), from the Saber-tooth Cave local fauna, Citrus County, Florida. Simpson's material included a rostral fragment with an incisor, premolar, and first molar. The Saber-tooth Cave local fauna is regarded by Kurten (1965:219) as having been recovered from Sangamon deposits. _Thomomys_ is unknown from Wisconsin deposits in the eastern United States, and today the genus does not occur east of the Great Plains.

_Thomomys_ of Rancholabrean provincial age from the western United States and México is known only from Wisconsin beds.

Three extinct species of _Thomomys_, all referable to the subgenus _Thomomys_, have been described. _Thomomys microdon_ Sinclair (1905:146), based on the rostral portion of a skull without a mandible, is from the Potter Creek Cave local fauna, Shasta County, California, and has been recovered also from Samwel Cave, Shasta County, California. _T. microdon_ closely resembles _Thomomys monticola_ that lives in the area today. _Thomomys scudderi_ Hay (1921:614) is from the Fossil Lake (or Christmas Lake) local fauna in central Oregon. Elftman (1931:10-11) referred these specimens to _Thomomys townsendii_, and he considered _T. scudderi_ to be a synonym of _T. townsendii_. Davis (1937:156-158) disagreed with Elftman concerning the taxonomic status of _T. scudderi_, which he regarded as a valid species. According to Davis, _T. scudderi_ is more closely allied to _Thomomys bottae_ than to _T. townsendii_. Cope (1878:389; 1889:160-165) had referred the same specimens to _Thomomys clusius_ (now _Thomomys talpoides clusius_). Cope considered the beds to be Pliocene in age. In all accounts of the Fossil Lake local fauna up to Hay (1921), the specimens of _Thomomys_ were referred to the species _clusius_, _talpoides_, or _bulbivorus_ (see Elftman, _loc. cit._). The Fossil Lake local fauna is currently considered as being of Rancholabrean provincial age, probably dating from the Wisconsin glacial maximum when the lake reached its greatest size. The third extinct species described from the Wisconsin is _Thomomys vetus_ Davis (1937:156), also from the Fossil Lake local fauna in Lake County, Oregon. Davis pointed out that _T. vetus_ differs from _T. scudderi_ Hay, of the same fauna, in larger size and other cranial details, and that it is closely allied to the living species _Thomomys townsendii_, and not to _Thomomys talpoides_, which is the only species of _Thomomys_ living in the area today.

_Thomomys townsendii_ was recovered by Gazin (1935:299) from the American Falls beds (probably Wisconsin deposits) in Idaho.

_Thomomys talpoides_ is reported from the Howard Ranch local fauna in Hardeman County, western Texas, by Dalquest (1965:69-70), who referred the isolated teeth to _T. talpoides_ on geographic grounds, apparently on the erroneous assumption that _T. talpoides_ was the species of _Thomomys_ nearest geographically to Hardeman County. Hay (1927:259) reported _Thomomys fuscus_ [= _Thomomys talpoides_] from late Pleistocene beds near Wenatchee, Chelan County, Washington. Hibbard (1951:229) recorded _Thomomys talpoides_ from late Pleistocene deposits in Greeley County, Kansas, and Walters (1957:540) reported the same species from late Pleistocene deposits in Clark County, Kansas. According to Hibbard (1958:14) other remains reported as _T. talpoides_ have been recovered from numerous areas of Wisconsin glacial drift in western North America.

_Thomomys bottae_ has been identified from Wisconsin age deposits in western North America, as follows: Burnet Cave, Gaudalupe Mt., New Mexico (Schultz and Howard, 1935:280); Carpinteria Asphalt, California (Wilson, 1933a:70); McKittrick Asphalt, Kern County, California (J. R. Schultz, 1938:206); Rancho La Brea, Los Angeles County, California (Dice, 1925:125--specimens described as a new subspecies, _T. b. occipitalis_); Papago Springs Cave, Santa Cruz County, Arizona (Skinner, 1942:150 and 158--probably _bottae_, but possibly _umbrinus_ on the assumption that the two are specifically instead of subspecifically distinct); Isleta Cave, Bernalillo County, New Mexico (Harris and Findley, 1964:115--some of these fossils may be post-Wisconsin in age); Potter Creek Cave and Samwel Cave, Shasta County, California (Sinclair, 1905:146--identified as _T. leucodon_, now a subspecies of _T. bottae_; also see Hay, 1927:214-215).

_Thomomys umbrinus_ has been reported from San Josecito Cave, Nuevo León, México (Russell, 1960:542); Upper Bercerra, México (Hibbard, 1955a:51--identified only as _Thomomys_ sp., but undoubtedly referable to _T. umbrinus_). Post-Wisconsin remains of _Thomomys umbrinus_ are reported by Alvarez (1964:6) from capa II and capa III of the Cueva La Nopalera, southwestern Hidalgo. Hay (1927:222-223) reported specimens of the genus _Thomomys_ from Wisconsin deposits in Hawver Cave, Eldorado County, California, but did not assign them to species. Gilmore (1947:158) found the remains of _Thomomys umbrinus_ in cave deposits near Quatro Ciénegas in central Coahuila. These cave deposits may have been laid down during the Wisconsin, but more likely accumulated in the post-Wisconsin.

_Zygogeomys_

Remains found in the Curtis Ranch local fauna, Cochise County, in southeastern Arizona are regarded as of middle Pleistocene age. See Gazin (1942:481-484), Wilson (1937:39-40), Hibbard (1958:25), and Hibbard _et al._ (1965:510-511). Although some question as to the exact age of the Curtis Ranch local fauna still seems to exist, most authorities on the Pleistocene agree that the age is not Pliocene and that it is older than Rancholabrean. Gidley (1922:122) described the pocket gopher found in the Curtis Ranch beds as _Geomys parvidens_, which is preoccupied by _Geomys parvidens_ Brown (1908:194), a name proposed for the pocket gopher from the Conard Fissure of Arkansas; therefore, Hay (1927:136) proposed the name _Geomys persimilis_ for the Curtis Ranch species to replace _Geomys parvidens_ Gidley. _Geomys persimilis_ Hay became the type species of Gazin's genus _Nerterogeomys_ (1942:507). In this paper, _Nerterogeomys_ is considered to be a junior synonym of _Zygogeomys_.

_Zygogeomys persimilis_ is represented by a rostral fragment bearing all the cheek teeth on the left side and the upper incisors. In addition, two lower jaws, one with the first three cheek teeth, are referred to the species (see Gazin, 1942:507). The fossils identified as _Geomys_ from the Arroyo San Francisco, Cedazo fauna, in Aguascalientes, México, by Mooser (1959:413) may be referable instead to _Zygogeomys_. I have not seen the specimens and no figures are available; Mooser states that a cranium was recovered. If either the upper premolar or third molar is in place, generic identification could be made with reasonable certainty. No other fossils of _Zygogeomys_ have been uncovered in late Pleistocene deposits and the significance of the absence of _Zygogeomys_ has been discussed in an earlier paragraph of this section. _Geomys_ has not been found so far south as Aguascalientes, but _Zygogeomys_ occurs farther south now and presumably had a more extensive range on the plateau to the north in the Pleistocene.

_Geomys_

_Geomys_ is common in Pleistocene deposits, especially on the Great Plains. Certainly the center of differentiation for _Geomys_ was in this region, although at times, probably when conditions were favorable, _Geomys_ expanded its range into adjacent areas, reaching the Pacific Coast in Irvingtonian times and the Atlantic Coast at the time of the Illinoian glaciation. The earliest Pleistocene records of the genus are from the Great Plains. McGrew (1944:49) described _Geomys quinni_ from the Sand Draw local fauna, Brown County, Nebraska, considered by Hibbard (1958:11) to be Nebraskan in age. As mentioned in the account of Pliocene geomyids, _Geomys quinni_ occurs also in the late Pliocene deposits of southwestern Kansas. Also, _Geomys quinni_ occurs in the Broadwater-Lisco local fauna of Morrill and Garden counties, western Nebraska (Barbour and Schultz, 1937:3; Schultz and Stout, 1948:560-563; Schultz _et al._, 1951: table 1). The Broadwater-Lisco is currently regarded as Aftonian deposits (Schultz and Stout, _loc. cit._; Hibbard, 1958:11). Hibbard (1956:174) identified _Geomys quinni_ from the Deer Park local fauna, probably deposited during the early Aftonian interglacial, of Meade County, Kansas. Strain (1966:36) described _Geomys paenebursarius_ on the basis of fossils obtained from early Pleistocene deposits of the Hudspeth local fauna from western Hudspeth County in the Trans-Pecos of Texas. The Hudspeth fossils were probably deposited during the Aftonian interglacial. From Kingman County, Kansas, Hibbard (_op. cit._: 164) recovered isolated teeth of _Geomys_ from the Dixon local fauna, regarded by him (_op. cit._:153-154) as deposited during the latest Nebraskan glaciation, and correlated by him with the Sand Draw local fauna of Nebraska. Hibbard (1958:11) later regarded the Dixon as a transitional fauna between Nebraskan and Aftonian. The remains of _Geomys_ from the Dixon are known only from isolated teeth. The teeth are small, and suggest that a smaller species of _Geomys_ may have occurred along with the more common and larger _G. quinni_ during the early Pleistocene (see discussion beyond of the Saunders _Geomys_). _Geomys quinni_ was widespread and common throughout the central Great Plains from the late Pliocene (Rexroad fauna) through the early Pleistocene (Nebraskan and Aftonian deposits).

Hibbard (1956:179) referred the pocket gopher remains taken from the Saunders local fauna in Meade County, Kansas, to _Geomys tobinensis_, a small species having continuous enamel bands around the lower premolar in younger specimens. The Saunders local fauna was deposited in the late Aftonian and is younger than the Deer Park local fauna discussed above. Paulson (1961:138) later pointed out that the Saunders _Geomys_ is distinct from _Geomys tobinensis_; hence, the small pocket gopher from the Saunders local fauna is probably an unnamed species, perhaps more closely allied to _paenebursarius_ than to _quinni_. The small _Geomys_ reported from the Aftonian Broadwater-Lisco local fauna of Nebraska (Schultz and Stout, 1948:563) may also be the same as the Saunders pocket gopher, but the smaller adult specimens occurring in the same bed with larger specimens probably are females and the larger specimens males. In all living Geomyini females have smaller skulls than males.

The Irvingtonian provincial age is currently regarded as Middle Pleistocene and includes the late Kansan glaciation (that part occurring after the glacial maximum) and the Yarmouthian interglacial (see Hibbard _et al._, 1965:512-514). The Irvingtonian provincial age, therefore, follows the late Blancan provincial age of the early Pleistocene and is succeeded by the Rancholabrean provincial age of the late Pleistocene. No specimen of an Irvingtonian _Geomys_ is referable to any living species. Two Irvingtonian species have been described. Hibbard (1944:735) named _Parageomys tobinensis_ [= _Geomys tobinensis_] from the Tobin local fauna of Russell County, Kansas. This species since has been reported from the Cudahy local fauna of Meade County, Kansas (Paulson, 1961:137). Hibbard (1956:183) also identified as _Geomys tobinensis_ the pocket gopher recovered from the Saunders local fauna, a late Aftonian deposit of Meade County, Kansas, and reduced the technical name _Parageomys_ from generic to subgeneric rank. Paulson (_op. cit._:138) pointed out that the Saunders specimens differ from _G. tobinensis_, and he, therefore, restricted the name to the small _Geomys_ of the Cudahy and Tobin local faunas of Irvingtonian provincial age. _G. tobinensis_ is markedly smaller than the Blancan _G. quinni_. The Cudahy and Tobin local faunas are of approximately the same age, and presently both are included in one unit, the Cudahy fauna. The Cudahy fauna is considered to have been deposited in late Kansan as it occurs in strata immediately below the Pearlette ash.

Recently, White and Downs (1961:8) described a new Irvingtonian species, _Geomys garbanii_, from the middle Pleistocene Vallecito Creek local fauna of San Diego County, California. Many well preserved fossils of the new species were recovered. _Geomys garbanii_ is of medium size (approximately the size of one of the larger subspecies of _G. bursarius_), and significantly larger than the Irvingtonian _Geomys tobinensis_ of the Great Plains. The Vallecito Creek occurrence of _Geomys_ is the first authenticated record from the Pacific Coast region. Matthew (1902:320) erroneously referred remains of _Thomomys_ to the genus _Geomys_ in his revised list of Cope's earlier report on the Fossil Lake (or Silver Lake) fauna (see discussion of _Thomomys_ above).

A number of Irvingtonian fossil remains of _Geomys_ have not been identified with particular species. Hibbard (1941a:206) found _Geomys_ in the Borchers local fauna (deposited in the time of the Yarmouthian interglacial) of Meade County, Kansas. Also, _Geomys_ has been reported from several sites in Nebraska. Schultz and Tanner (1957:67) reported _Geomys_ from the Angus fossil quarry in Nuckolls County, south-central Nebraska. The Angus fossils were found in sediments of the Sappa Formation considered by Schultz and Tanner to be a Yarmouthian deposit. Fossil quarries (Hay Springs, Rushville, and Gordon) along the south side of the Niobrara River Valley in Sheridan County, Nebraska, have also provided records of geomyids. Both a large and small species of _Geomys_ have been reported from the more recently excavated Rushville and Gordon sites (Schultz and Stout, 1948:562-567, and table 3). In view of the great disparity in size owing to sex, these may actually be males and females of the same species, as mentioned above. The name Hay Springs has been used in reference to all three sites. The ages of the Hay Springs sites are approximately the same, but their correlation is presently under debate. Schultz and Tanner (1957:68-71) maintain that the fossils are distinctly middle Pleistocene, and that they were deposited during late Kansan glaciation, or perhaps from early Yarmouthian into early Illinoian, with the largest concentration coming from the Sappa sands of pre-Illinoian (Yarmouth) age. Hibbard (1958:25), basing his opinion on the presence of _Microtus pennsylvanicus_, and the stage of evolution of other species in the assemblage, regards the Hay Springs sites as probably Illinoian deposits, but certainly no older than that.

Mooser (1959:413) identified as _Geomys_ the pocket gopher from Irvingtonian deposits in Arroyo San Francisco (loc. no. 5) near the city of Aguascalientes, México. As suggested elsewhere in this account, these fossils may be referable to _Zygogeomys_ rather than _Geomys_. The Irvingtonian provincial age of this fauna was established by Hibbard and Mooser (1963:245-250). Other alleged occurrences have recently been compiled by Alvarez (1965:19-20). Maldonado-Koerdell (1948:20) noted four fossil occurrences of the genus _Geomys_ in México. Two of these from San Josecito Cave in Nuevo León have since been identified with the genera _Orthogeomys_ and _Pappogeomys_ (Russell, 1960:543-548); the third listed by Maldonado-Koerdell from "near Ameca, Jalisco," was based on Brown's (1912:167) mention of some bones supposedly of the family "Geomyidae," and the fourth refers to pocket gopher remains from the "Hochtals von Mexiko" listed as _Geomys_ by Freudenberg (1921:139). His generic identification is doubtful and the specimens should be compared with Mexican genera of the Geomyinae.