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+*** START OF THE PROJECT GUTENBERG EBOOK 40282 ***
+
+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
+
+[Illustration: Look for the Union label]
+
+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.
+
+    [Illustration: FIG. 1. Types of skulls in the subfamily
+       Geomyinae. × 1.
+
+    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.
+
+    [Illustration: FIG. 2. Probable distribution of the Subfamily
+       Geomyinae in the early Pleistocene (late Blancan), depicting
+       major areas of differentiation of the modern genera.
+
+    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.
+
+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
+
+  ===============+==============+==================+================
+   Baird 1858    | Gill 1872    | Winge 1887       | Merriam 1895
+                 | Coues 1877   | and 1924         | Ellerman 1940
+  ---------------+--------------+------------------+----------------
+   Family        | Family       | Family           | Family
+     Saccomyidae |   Geomyidae  |   Saccomyidae    |   Geomyidae
+  ---------------+--------------+------------------+----------------
+   Subfamily     |              | "Group"          |
+     Geomyinae   |              | Geomyini         |
+  -- -- -- -- -- +-- -- -- -- --+-- -- -- -- -- -- +-- -- -- -- -- -
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+  _Thomomys_     | _Thomomys_   | _Thomomys_       | _Thomomys_
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              |                  | _Zygogeomys_
+                 |              |                  |
+                 |              |                  |
+  _Geomys_       | _Geomys_     | _Geomys_         | _Geomys_
+                 |              |                  |
+                 |              |                  | _Orthogeomys_
+                 |              |                  | _Heterogeomys_
+                 |              |                  | _Macrogeomys_
+                 |              |                  |
+                 |              |                  | _Pappogeomys_
+                 |              |                  | _Cratogeomys_
+                 |              |                  | _Platygeomys_
+  -- -- -- -- -- +-- -- -- -- --+-- -- -- -- -- -- +-- -- -- -- -- -
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              | *_Pleurolicus_   |
+                 |              |                  |
+                 |              |                  |
+                 |              | *_Entoptychus_   |
+  ---------------+--------------+------------------+----------------
+                 |              |                  |
+                 |              |                  |
+                 |              |                  |
+                 |              | "Group"          |
+                 |              | Gymnoptychine**  |
+                 |              | _Gymnoptychus_   |
+  ---------------+--------------+------------------+----------------
+  Subfamily      | Family       | "Group"          |
+    Saccomyinae  |  Saccomyidae |  Saccomyini      |
+  -------------+-------------+------------------+----------------
+
+  =====================+=====================+===================
+   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.
+
+_Chronologic and geographic range._--Known from late middle Pliocene
+deposits to Recent. The range of living members extends from extreme
+southern Manitoba and the southeastern United States south to southern
+Panamá, and probably northern Colombia, South America.
+
+_Diagnosis._--Size small to large (condylobasal length of skull 33.0
+to 73.0 in adults, including both sexes); sexual dimorphism marked,
+sometimes strongly, females being smaller than males, especially in
+cranial dimensions; upper incisors invariably grooved, number and
+position of grooves varying according to genus; cheek teeth
+high-crowned and ever-growing, except in one primitive genus
+(_Pliogeomys_); all three lower molars and M1 and M2 monoprismatic,
+and elliptical in cross-section in final stages of wear (teeth of
+young, subadult, and adult animals); primitive biprismatic patterns
+(as known from Recent specimens) occurring only in pre-final stages of
+wear (teeth of juveniles only); biprismatic patterns of lower molars
+as in _Dikkomys_, and upper molars as in _Pliosaccomys_ (for detailed
+description of these patterns, see account beyond of the phylogeny of
+the Geomyinae); m3 becoming monoprismatic, anteroposteriorly
+compressed and elliptical in cross-section like m1 and m2, but M3
+remaining, with rare exceptions (see accounts of _Geomys_ and
+_Pappogeomys_ beyond), at least partially biprismatic throughout life,
+having one or both lateral inflections usually persisting (with
+exceptions) and developing various occlusal shapes (subtriangular,
+elongate, obcordate, suborbiculate, or quadriform) but never
+elliptical.
+
+Enamel of cheek teeth reduced to interrupted plates, with exception of
+p4 in _Pliogeomys_; plate on posterior wall of P4 variable, occurring
+completely across posterior surface in primitive members, but
+progressively reduced to lingual side only or completely lost in
+modern genera (see generic accounts beyond for detailed description);
+both anterior and posterior plates usually retained in M1 and M2,
+posterior plate sometimes reduced to lingual side or completely lost
+(as in _Pappogeomys_) but anterior plate always completely retained;
+M3 usually having three plates, one anterior and two lateral;
+posterior plate wanting (sometimes lingual plate moved to posterior
+position); plates retained completely across posterior walls of all
+lower cheek teeth with no reduction, but anterior plates of m1-3
+always lacking, except in primitive genus _Pliogeomys_ (only Geomyini
+having both anterior and posterior enamel plates on lower molars).
+
+Skull primitively generalized, but becoming specialized towards either
+dolichocephaly (_Orthogeomys_) or platycephaly (_Pappogeomys_) in two
+modern genera; skull highly specialized for fossorial life; mandible
+stout and deep, angular process being high and diverging laterally at
+right angles to ramus; masseteric ridge and fossa weakly developed in
+primitive members, becoming well developed and massive in modern
+genera; basitemporal fossa absent in primitive forms (_Pliogeomys_ and
+early members of _Zygogeomys_); pelage usually soft, but harsh and
+hispid in some genera; forefeet broad and massive, claws long and
+stout for digging; body form remarkably fossorial.
+
+The tribe Geomyini includes the most highly specialized members of the
+subfamily Geomyinae.
+
+
+Key to the Genera of the Tribe Geomyini
+
+  A  Cheek teeth rooted; p4 with uninterrupted enamel loop; enamel
+     plates on both anterior and posterior walls of m1 and m2;
+     masseteric ridge weakly developed, low, not massive.
+     Genus _Pliogeomys_                                         p. 522
+
+  A´ Cheek teeth rootless, ever-growing; p4 with enamel investment
+     interrupted at ends of columns, consequently, forming four
+     isloted plates; enamel plate retained only on posterior wall
+     of m1 and m2, anterior wall without trace of enamel (except
+     rarely in pre-final stage of wear in _Geomys tobinensis_ of
+     middle Pleistocene); masseteric crest strongly developed and
+     massive.
+
+     B  Enamel plate on posterior wall of P4, but usually
+        restricted to lingual end of tooth (usually absent in
+        subgenus _Orthogeomys_ of genus _Orthogeomys_);
+        M3 conspicuously bicolumnar, longer than wide owing to
+        elongation of posterior loph.
+
+        C  Upper incisor bisulcate; skull generalized; rostrum
+           relatively narrow; length of labial enamel plate of
+           M3 decidedly less than length of lingual plate;
+           pelage soft and thick. Genus _Zygogeomys_            p. 523
+
+        C´ Upper incisor unisulcate; skull strongly
+           dolichocephalic; rostrum remarkably broad and massive;
+           length of lingual plate of M3 approximately equal to,
+           or greater than, length of labial plate; pelage harsh,
+           often hispid and scant. Genus _Orthogeomys_          p. 528
+
+     B´ Posterior wall of P4 without trace of enamel; M3 not
+        strongly bicolumnar, having shallow re-entrant fold on
+        labial side, and crown no longer than wide owing to
+        shortness of posterior loph.
+
+           D  Upper incisor bisulcate; skull generalized; both
+              anterior and posterior walls of M1 and M2 having
+              complete enamel plates. Genus _Geomys_            p. 525
+
+           D´ Upper incisor unisulcate; skull generalized or
+              tending towards platycephaly; enamel plate on
+              posterior wall of M1 usually reduced to lingual
+              side or absent (complete only in one species,
+              _Pappogeomys bulleri_); enamel plate on posterior
+              wall of M2 also absent in advanced species
+              (subgenus _Cratogeomys_). Genus _Pappogeomys_     p. 532
+
+
+Genus =Pliogeomys= Hibbard
+
+    1954. _Pliogeomys_ Hibbard, Michigan Acad. Sci., Arts and
+          Letters, 39:353.
+
+_Genotype._--_Pliogeomys buisi_ Hibbard, 1954, from Buis Ranch local
+fauna (middle Pliocene), Beaver County, Oklahoma.
+
+_Chronologic range._--Latest Middle Pliocene, known only from the
+highest part of the Hemphillian mammalian fauna (Buis Ranch local
+fauna, Oklahoma). Professor Hibbard informs me (personal
+communication) that he found the type, a right ramus, lying on the
+surface near the base of the fossil beds. The isolated teeth of small
+geomyids from the Saw Rock Canyon local fauna (see Hibbard, 1953:392)
+may also be referable to this genus. The Saw Rock Canyon local fauna
+may also be middle Pliocene in age but is considered to be from the
+later part of the late Pliocene, and, therefore, somewhat younger than
+the Buis Ranch local fauna (Hibbard, _op. cit._:342).
+
+_Description and discussion._--The size of members of this small genus
+of the Geomyinae is about the same as in smaller adults of _Geomys
+bursarius_. According to Hibbard (_op. cit._:353), the holotype is
+smaller than specimens from the Rexroad local fauna referred to
+_Geomys quinni_ and larger than specimens referred to _Zygogeomys_ cf.
+_minor_. The cheek teeth are rooted, and the crowns are as high as
+those of living geomyids. The upper incisor is bisulcate, and the
+inner groove is fine and indistinct in places.
+
+Of the molariform dentition only the lower premolar and first two
+lower molars are known. The enamel investment of p4 is complete, and
+would not be subject to interruption at any stage of wear; the two
+prisms are joined at their mid-points, and the isthmus of dentine is
+relatively broad (as in _Pliosaccomys_) when compared with modern
+pocket gophers of this tribe. Also, the re-entrant folds, rather than
+having parallel sides, diverge broadly to the sides. The divergence is
+especially noticeable in the labial fold. The lower deciduous premolar
+would have formed essentially the same enamel pattern with wear as
+observed in _Nerterogeomys_ [= _Zygogeomys_] cf. _minor_ (see Hibbard,
+1954:fig. 5, A and B) and _Pliosaccomys dubius_ (see Wilson, 1936; pl.
+1, fig. 1). Each molar is a single column in the final stages of wear;
+pre-final stages are unknown. Anterior and posterior enamel plates are
+present on m1 and m2 (m3 has not been recovered). The dentine tracts
+of m1 are exposed over a relatively wide surface; therefore, the
+enamel plates are distinctly separated. The tracts of dentine of m2
+are much narrower than in m1 and the enamel plates are barely
+separated at the anterolateral margin of the tooth. Possibly the
+enamel band of m2 was continuous in an earlier stage of wear.
+
+The mandible is stout and its general construction not unlike that in
+modern geomyines. The capsule at the base of the angular process that
+receives the terminal end of the lower incisor is well developed. The
+base of the angular processes is preserved, and suggests that the
+process was short and decidedly smaller than in living examples of the
+tribe. The masseteric ridge is distinct but weakly developed, and not
+at all massive as in living pocket gophers. The mental foramen is
+immediately anterior, and slightly ventral, to the anterior extension
+of the crest. The basitemporal fossa is absent as such, but its
+position is marked by a slight depression.
+
+_Specimens examined._--Two rami; nos. 29147 (holotype) and 33446;
+several isolated teeth 30194 and 30195, including an upper incisor and
+a dp4 (deciduous lower premolar), all from Univ. Michigan Mus. Paleo.
+
+_Referred species._--One.
+
+    *_Pliogeomys buisi_ Hibbard, 1954. Papers Michigan Acad. Sci.,
+        Arts, and Letters, 39:353. Type from Buis local fauna, latest
+        middle Pliocene, Beaver County, Oklahoma.
+
+
+Genus =Zygogeomys= Merriam
+
+    1895. _Zygogeomys_ Merriam, N. Amer. Fauna, 8:195, January 31.
+
+    1942. _Nerterogeomys_ Gazin, Proc. U. S. Nat. Mus., 92:507
+          (type, _Geomys persimilis_ Hay, 1927).
+
+_Type._--_Zygogeomys trichopus_ Merriam, 1895, from Nahuatzen,
+Michoacán.
+
+_Chronologic range._--Late Pliocene (Benson and Curtis Ranch local
+faunas, Arizona, and ?Rexroad Formation, Kansas) to Recent.
+
+_Description and discussion._--The size is small to medium for the
+subfamily Geomyinae. This genus is distinguished principally by the
+retention of primitive features. In the living species, the skull is
+generalized, rather than specialized toward either extreme
+dolichocephaly or platycephaly. The angular process is short, barely
+exceeding the lateral extensions of the mastoid process of the
+squamosal. The rostrum is remarkably narrow in relation to its length.
+The jugal is reduced and displaced ventrally, causing the maxillary
+arm of the zygomata to articulate with the squamosal arm of the
+zygomata along the dorsal border of the zygomatic arch (a feature
+observed also in _Orthogeomys cherriei costaricensis_).
+
+The upper incisor, recovered in material from the late Pliocene and
+middle Pleistocene, is bisulcate as in the genus _Geomys_ and the
+primitive genus _Pliogeomys_. The enamel plate across the posterior
+wall of P4 is either complete (late Pliocene to late Pleistocene) or
+restricted to the lingual half of the tooth (always restricted in
+living species). The Pliocene specimens of the Rexroad local fauna
+referred to _Nerterogeomys_ cf. _minor_ by Hibbard (1950:138-139) are
+exceptional. In these specimens the length and position of the
+posterior enamel plate is variable; however, all but one specimen had
+persistant enamel. Evidently, in approximately 43 per cent of the
+specimens, a complete enamel blade was present (see Paulson,
+1961:139), and in the others (except the one without any enamel) the
+plate was restricted to a small area of the ventral surface, usually
+on the lingual side of the loph. Hibbard suggested that the decrease
+in size of the plate, and its restriction to the lingual side, may be
+a function of age. Hence, most adults would be characterized by the
+reduced posterior plate on the upper premolar. Although age may be the
+important factor, intragroup variation cannot be ruled out. It is of
+interest to note that in all specimens from the Benson (type series of
+_P. minor_) and Curtis Ranch local faunas, the former of late Pliocene
+age and the latter of middle Pleistocene age, the enamel plates are
+complete on the posterior face of the upper premolar. As mentioned
+before, the specimens from Kansas may actually represent the
+transitional stages of the early evolution of _Geomys_ in which the
+posterior plate of P4 is entirely lost. The enamel pattern of p4 is
+like that in other members of the tribe (excepting the genus
+_Pliogeomys_). The re-entrant angles of P4 and p4 are widely open
+(obtuse) in the examples recovered from late Pliocene and middle
+Pleistocene deposits, representing retention of a trait that is
+primitive in the Geomyini (see account of phylogeny).
+
+M1 and M2 are elliptical in cross-section and each has an enamel plate
+on both the anterior and posterior surface. In the living species (_Z.
+trichopus_), the posterior enamel plate fails to reach the labial
+margin of the tooth and is restricted to the lingual two-thirds of the
+posterior surface; however, the enamel plates are complete in the late
+Pliocene species (_Z. minor_) and the middle Pleistocene species (_Z.
+persimilis_), being only slightly separated from the anterior plate by
+narrow tracts of dentine on the ends of the tooth. M3 is partly
+biprismatic in the living species, the two incompletely divided lophs
+being separated by a distinct outer sulcus. The posterior loph is
+elongated and forms a conspicuous heel paralleling the evolution of
+this trait in the genus _Orthogeomys_; therefore, the crown is longer
+than wide. The posterior part of the tooth is protected by two lateral
+enamel plates; of the two, the lingual plate is especially long and
+extends to the end of the heel. M3 has not been recovered in the
+Pliocene species, but in the middle Pleistocene species (_Z.
+persimilis_) M3 is subtriangular, no longer than wide, and the lateral
+inflections are weakly developed. The trend towards elongation of M3
+evidently occurred in late Pleistocene evolution of the genus. All
+three of the inferior molars are elliptical, and only the posterior
+enamel plate is present (as in all other genera of the tribe except
+_Pliogeomys_).
+
+The masseteric ridge of the mandible is well developed. In the late
+Pliocene species _Z. persimilis_ and _Z. minor_ the mental foramen is
+directly beneath the anterior extension of the masseteric ridge, but
+in the living species, _Z. trichopus_, the foramen lies well anterior
+to the ridge. The basitemporal fossa in the living species is well
+developed and deep; in the Pliocene species it is usually distinct but
+shallow (late Pliocene specimens of _Z. minor_).
+
+_Referred species._--Three (two extinct and one living; the last has
+two subspecies):
+
+    *_Zygogeomys minor_ (Gidley), 1922. U. S. Geol. Surv. Prof. Paper,
+           131:123, December 26. Type from Benson local fauna (late
+           Pliocene), Cochise County, Arizona; also known from the
+           Rexroad local fauna, Meade County, Kansas.
+
+    *_Zygogeomys persimilis_ Hay, 1927. Carnegie Inst. Washington
+           Publ., 136. Originally described by Gidley, 1922 (U. S. Geol.
+           Surv. Prof. Papers, 131:123, December 26) as _Geomys
+           parvidens_ which was preoccupied by _G. parvidens_ Brown,
+           1908. Type from Curtis Ranch local fauna (middle
+           Pleistocene), Cochise County, Arizona.
+
+    _Zygogeomys trichopus trichopus_ Merriam, 1895. N. Amer. Fauna,
+           8:196, January 31. Type from Nahuatzen, Michoacán.
+
+    _Zygogeomys trichopus tarascensis_ Goldman, 1938. Proc. Biol. Soc.
+           Washington, 51:211, December 23. Type from 6 mi. SE
+           Pátzcuaro, 8,000 ft., Michoacán.
+
+
+Genus =Geomys= Rafinesque
+
+    1817. _Geomys_ Rafinesque, Amer. Monthly Mag., 2(1):45, November.
+
+    1817. _Diplostoma_ Rafinesque, Amer. Monthly Mag., 2(1):44-45,
+          November. Included species: _Diplostoma fusca_ Rafinesque
+          [= _Mus bursarius_ Shaw] and _Diplostoma alba_ Rafinesque
+          [= _Mus bursarius_ Shaw] from the Missouri River region.
+
+    1820. _Saccophorus_ Kuhl, Beitr. Zool. und Vergl. Anat., pp. 65, 66.
+          Type: _Mus bursarius_ Shaw, from upper Mississippi Valley.
+
+    1823. _Pseudostoma_ Say, Long's Expd. Rocky Mts., I, pp. 406. Type:
+          _Pseudostoma bursaria_ [= _Mus bursarius_ Shaw], from upper
+          Mississippi Valley.
+
+    1825. _Ascomys_ Lichtenstein, Abh. K. Akad. Wiss. Berlin (1822),
+          p. 20., fig. 2. Type: _Ascomys canadensis_ Lichtenstein
+          [= _Mus bursarius_ Say], probably from upper Mississippi Valley.
+
+    1944. _Parageomys_ Hibbard, Bull. Geol. Soc. Amer., 55:735, June.
+          Type: _Parageomys tobinensis_ Hibbard, from Pleistocene, Cudahy
+          (Tobin) local fauna, Russell Co., Kansas.
+
+_Type._--_Geomys pinetis_ Rafinesque, 1817, restricted to Screven
+County, Georgia, in region of the pines.
+
+_Chronologic range._--Late Pliocene faunas of Blancan age (Rexroad,
+Kansas, and Sand Draw, Nebraska, local faunas) to Recent. Reported
+from numerous Pleistocene deposits of all stratigraphic levels,
+especially from the Great Plains, where common today.
+
+_Description and discussion._--Pocket gophers of this genus are
+medium-sized geomyids; none is so small as the average-sized
+_Thomomys_. The skull is generalized and lacks the dolichocephalic and
+platycephalic specializations seen in the genera _Orthogeomys_ and
+_Pappogeomys_, respectively. _Geomys_ closely resembles _Zygogeomys_,
+but retains fewer of the primitive characters of the ancestral stock.
+At the same time, _Geomys_ has several specializations. Even so, a
+considerable amount of parallelism is evident in the phyletic trends
+of the two genera.
+
+The upper incisor of _Geomys_ is bisulcate as in _Pliogeomys_ and
+_Zygogeomys_; the deeper grove is medial and the shallower grove lies
+near the inner border of the tooth. The premolar, above and below, is
+bicolumnar; and two columns are joined at their mid-points (deep
+re-entrant angles separate the columns at the sides). A permanent
+enamel plate protects the anterior face of the anterior loph, and
+enamel bands outline each of the re-entrant folds. In p4 a complete
+enamel plate covers the posterior surface of the posterior loph. All
+of the enamel bands are interrupted by tracts of dentine, except in
+the initial stages of wear of the occlusal surface of the newly
+erupted tooth. For a short time in living _Geomys_, the enamel bands
+are continuous as observed in juveniles of _Geomys bursarius major_
+(KU 5628, 8531, and 41540). But, the enamel cap is thin and the
+dentine tracts, which are high on the sides of the tooth, are soon
+revealed by a minimum of wear on the crown. Therefore, the adult, or
+final, pattern characterized by interrupted enamel plates emerges
+early in life and remains throughout the life of the individual.
+Evidence from fossil _Geomys_, especially from specimens from early
+and late Pleistocene deposits, suggests that the final adult pattern
+appears later, ontogenetically, than in Recent specimens. Some of the
+fossil premolars in initial stages of wear have continuous and
+uninterrupted bands of enamel. _Geomys quinni_ of the late Pliocene
+and early Pleistocene has the interrupted pattern seen in late
+Pleistocene and Recent _Geomys_. Also, in late Pliocene and early
+Pleistocene species, the re-entrant folds diverge laterally and form
+"open" angles. In later taxa (middle Pleistocene to Recent) the folds
+are compressed and parallel-sided, and the "open" folds are found only
+in the early stages of wear.
+
+The posterior enamel plate of P4 disappears in the final stages of
+wear as the interrupted enamel pattern is formed. In the late
+Pleistocene and Recent _Geomys_, the loss of the posterior plate
+occurs early in life, usually in the first phases of wear on the
+occlusal surface of the newly erupted tooth, but in fossils of
+_Geomys_ of corresponding ontogenetic age from the early and middle
+Pleistocene, the posterior plate is retained in some individuals until
+a later phase of wear, thereby delaying the appearance of the final
+pattern. Indeed, in five or fewer per cent of the individuals (see
+Paulson, 1961:138-139; and White and Downs, 1961:18) a vestige of
+enamel is retained throughout life or at least until late in
+adulthood. In _Geomys tobinensis_, for example, a thin, but
+transversely complete, plate of enamel occurs all the way down to the
+base of the loph (Paulson, _loc. cit._) and would persist throughout
+life. In _Geomys garbanii_, a vestige on the lingual side of the
+posterior surface of a fully adult specimen was noted by White and
+Downs (_loc. cit._). Vestiges of the posterior plate occur less
+frequently in living geomyids. Paulson (_loc. cit._) found a posterior
+plate in one of 75 specimens of _Geomys bursarius dutcheri_. A young
+(suture present between exoccipitals and supraoccipital) female of
+_Geomys pinetis austrinus_ (KU 23358) has a vestige of the posterior
+plate on the lingual side of the tooth as White and Downs (_loc.
+cit._) observed in a specimen of _Geomys garbanii_. The enamel, I
+suspect, tends to be thicker on the lingual than on the labial side of
+the loph and extends farther down the lingual surface in some
+individuals; therefore, wear on the occlusal surface erodes it down to
+the dentine more rapidly on the labial than on the lingual side. The
+tendency of enamel to be retained is a primitive feature.
+
+A lower molar of _Geomys_ is a single elliptical column, and enamel is
+restricted to the posterior surface as in _Zygogeomys_, _Orthogeomys_,
+and _Pappogeomys_. Paulson (_loc. cit._) found a thin enamel plate on
+the anterior surfaces of the lower molars in about five per cent of
+the individuals of _Geomys tobinensis_ from the Cudahy local fauna
+(middle Pleistocene, deposits of the late Kansan glaciation). An
+anterior plate is unknown in other members of the tribe Geomyini,
+except in the primitive genus _Pliogeomys_ of the middle Pliocene.
+Occurrence of the plate in _Geomys tobinensis_ is an atavistic trait.
+Primitive dental patterns occur occasionally in geomyids, as pointed
+out above, but the frequency of occurrence in _G. tobinensis_ is
+higher than would be expected.
+
+M1 and M2, like the lower molars, are elliptical in cross-section.
+Complete enamel plates on the anterior and posterior surfaces are
+separated by tracts of dentine on the sides of each tooth. M3 is
+usually suborbicular (sometimes subtriangular) in cross-section. The
+tooth is not especially elongated posteriorly and usually has no
+definite heel; therefore, it is not significantly longer than wide.
+Living species of _Geomys_ rarely have a well defined outer re-entrant
+fold on M3; less than 10 per cent of the individuals (and usually only
+one side in each individual in which it occurs) have it, although a
+shallow inconspicuous groove occurs more frequently. The biprismatic
+molar characteristic of the ancestral morphotype is less often found
+in _Geomys_ than in any other living member of the tribe Geomyini. The
+outer re-entrant fold and biprismatic pattern are more often present
+in the extinct species _Geomys garbanii_ of the Middle Pleistocene
+than in other species. Less than 24 per cent of the third upper molars
+in _Geomys garbanii_ lack a tract of the re-entrant fold and more than
+38 per cent have a well developed outer fold (see White and Downs,
+1961:13, 18). The bicolumnar pattern, although incomplete, would be
+clearly evident in those teeth having a well marked re-entrant fold;
+the pattern occurs less frequently in those teeth with no fold or only
+a slight one. M3 of geomyids is not usually recovered and, therefore,
+the occlusal pattern of M3 is unknown in most extinct kinds of
+_Geomys_. In Recent _Geomys_ the fold is more common in the eastern
+_pinetis_ species-group than in the western _bursarius_ species-group.
+
+The masseteric ridge on the outer side of the mandible is well
+developed in all species of the genus. The position of the mental
+foramen relative to the anterior part of the ridge varies with
+individuals and according to species. The basitemporal fossa is always
+present, but is shallower in the late Pliocene and Pleistocene species
+than in Recent species. The angular process is short.
+
+_Referred species._--The twelve species, five of which are extinct,
+are as follows:
+
+_quinni_ species-group
+
+    *_Geomys quinni_ McGrew, 1944. Geol. Ser., Field Mus. Nat. Hist.,
+        9 (546):49, January 20. Type from Sand Draw local fauna (late
+        Pliocene), Brown County, Nebraska; also known from
+        Broadwater-Lisco local faunas (early Pleistocene), Morrill and
+        Garden counties, Nebraska, Deer Park local fauna (early
+        Pleistocene), Meade County, Kansas.
+
+    *_Geomys paenebursarius_ Strain, 1966. Bull. Texas Memorial Mus.,
+        10:36. Type from Hudspeth local fauna (early Pleistocene),
+        Hudspeth County, Texas.
+
+    *_Geomys tobinensis_ Hibbard, 1944. Bull. Geol. Soc. Amer.,
+        55:736. Type from Tobin local fauna (middle Pleistocene),
+        Russell County, Kansas; also known from Cudahy local fauna
+        (middle Pleistocene), Meade County, Kansas.
+
+    *_Geomys garbanii_ White and Downs, 1961. Contrib. Sci., Los
+        Angeles Co. Mus., 42:1-34, June 30. Type from Vallecito Creek
+        local fauna (middle Pleistocene), San Diego County, California.
+
+    *_Geomys bisulcatus_ Marsh, 1871. Amer. Jour. Sci., 3:121. Type
+        from Loup River fossil beds, near Camp Thomas, Nebraska
+        (probably late Pleistocene).
+
+
+_bursarius_ species-group
+
+    *_Geomys parvidens_ Brown, 1908. Mem. Amer. Mus. Nat. Hist.,
+        9:194. (An extinct subspecies of _Geomys bursarius_ according to
+        White and Downs, 1961:6). Type from Conard Fissure local fauna
+        (late Pleistocene), northern Arkansas.
+
+    _Geomys bursarius_ (Shaw, 1800). Trans. Linn. Soc. London, 5:227.
+        Type from somewhere in Upper Mississippi Valley, North America.
+
+    _Geomys arenarius_ Merriam, 1895. N. Amer. Fauna, 8:139, January 31.
+        Type from El Paso, El Paso County, Texas.
+
+    _Geomys personatus_ True, 1889. Proc. U. S. Nat. Mus., 11:159,
+        January 5. Type from Padre Island, Cameron County, Texas.
+
+
+_pinetis_ species-group
+
+    _Geomys pinetis_ Rafinesque, 1806. Amer. Monthly Mag., 2 (1):45,
+        November. Type locality restricted to Screven County, Georgia.
+
+    _Geomys colonus_ Bangs, 1898. Proc. Boston Soc. Nat. Hist., 28:178,
+        March. Type from Arnot Plantation, about 4 mi. W St. Marys,
+        Camden County, Georgia.
+
+    _Geomys cumberlandius_ Bangs, 1898. Proc. Boston Soc. Nat. Hist.,
+        28:180, March. Type from Stafford Place, Cumberland Island,
+        Camden County, Georgia.
+
+    _Geomys fontanelus_ Sherman, 1940. Jour. Mamm., 21:341, August 13.
+        Type from 7 mi. NW Savannah, Chatham County, Georgia.
+
+
+Genus =Orthogeomys= Merriam
+
+    1895. _Orthogeomys_ Merriam, N. Amer. Fauna 8:172, January 31.
+
+    1895. _Heterogeomys_ Merriam, N. Amer. Fauna 8:179, January 31
+          (type, _Geomys hispidus_ Le Conte, 1862).
+
+    1895. _Macrogeomys_ Merriam, N. Amer. Fauna 8:185, January 31 (type,
+          _Geomys heterodus_ Peters, 1865).
+
+_Type._--_Geomys scalops_ Thomas, 1894, from Tehuantepec, Oaxaca,
+México.
+
+_Chronologic range._--Late Pleistocene Wisconsin deposits (San
+Josecito Cave local fauna, Nuevo León, México) to Recent.
+
+_Description and discussion._--Species of this genus are of medium to
+large size. The skull is strongly dolichocephalic in most species;
+the posterior part of the skull is especially narrow. The angular
+processes are remarkably short, especially in relation to the length
+of the mandible. The nasals and rostrum are relatively broad and
+heavy. The pelage is coarse, and often hispid. In some species the
+hairs are so sparsely distributed that the body appears almost naked,
+and none has so dense a covering of hair as do other genera. The genus
+occurs entirely within the tropical life-zones, and most of the
+external features seem to be associated with adaptation to tropical
+conditions.
+
+The upper incisor is unisulcate; the sulcus is usually near the inner
+border of the tooth, but in some species (subgenus _Orthogeomys_) it
+is more medial, and in a few individuals with an extremely wide groove
+the outer lip of the sulcus may actually reach the middle of the
+tooth. The groove is compressed or open. The premolar is a double
+column united at the mid-point. The two prisms are of approximately
+equal size, and the lateral re-entrant folds are so compressed that
+their sides are parallel. Enamel plates cover the anterior surface and
+border the re-entrant angles in both upper and lower premolars. As in
+other members of the tribe, the lower premolar has a fourth enamel
+plate on the posterior surface of the posterior lophid. In the upper
+premolar, the enamel plate is reduced to a narrow blade on the lingual
+side of the loph as in the living species of the genus _Zygogeomys_.
+In the subgenus _Orthogeomys_ the posterior plate is usually absent,
+and otherwise is narrow and near the lingual border of the tooth.
+
+Each lower molar, in the final stage of wear, consists of a single
+elliptical column having an enamel plate only on the posterior
+surface. The first and second upper molars are single elliptical
+columns having one enamel plate on the anterior surface and another
+on the posterior surface. The plates are separated by a tract of
+dentine on each side of the tooth. The third upper molar is partly
+bilophodont, and the two lophs are separated by a deep outer
+re-entrant fold. In many of the species an inner re-entrant fold also
+is retained, but in the adult tooth it is less distinct than the
+outer. In all of the species the posterior loph is long and forms a
+conspicuous heel; consequently the crown is significantly longer then
+wide. Moreover, the posterior loph has an enamel plate on each side.
+The labial plate always borders the outer re-entrant fold, and in the
+subgenus _Orthogeomys_ is infrequently separated into two small
+plates.
+
+The mandible is relatively long. Its masseteric ridge is well
+developed and massive. The basitemporal fossa is usually deep and well
+defined; it tends to be shallow in the subgenus _Orthogeomys_, and in
+young individuals is hardly more than a slight depression.
+
+
+Key to the Subgenera of _Orthogeomys_
+
+  A  Frontal wide and greatly inflated; no interorbital
+     constriction; enamel plate on posterior wall of P4 usually
+     absent, although sometimes having small plate, restricted
+     to lingual end of wall. Subgenus _Orthogeomys_             p. 529
+
+  A´ Frontal narrow and not greatly inflated; interorbital
+     region decidedly constricted; enamel plate on posterior wall
+     of P4 always present but short and restricted to lingual end
+     of wall.
+
+     B  Anterior margin of mesopterygoid fossa even with plane of
+        posterior wall of M3; postorbital bar weakly developed;
+        anteroposterior occlusal length of M3 equal to, or less
+        than, combined length of M1 and M2.
+        Subgenus _Heterogeomys_                                 p. 530
+
+     B´ Anterior margin of mesopterygoid fossa decidedly behind
+        plane of posterior wall of M3; postorbital bar strongly
+        developed; anteroposterior occlusal length of M3 more
+        than combined length of M1 and M2.
+        Subgenus _Macrogeomys_                                  p. 531
+
+
+Subgenus =Orthogeomys= Merriam
+
+    1895. _Orthogeomys_ Merriam, N. Amer. Fauna, 8:172, January 31.
+
+_Type._--_Geomys scalops_ Thomas, 1894, from Tehuantepec, Oaxaca,
+México.
+
+_Chronologic range._--Known only from the Recent.
+
+_Description._--Skull elongated and narrow (many skulls of nearly
+uniform breadth throughout), being extreme in dolichocephalic
+specializations; mandibles long and narrow, rami not spreading
+laterally, being more nearly parallel-sided than in other subgenera;
+angular processes short; breadth across zygomata not significantly
+exceeding breadth across mastoid processes (in many skulls
+considerably less); interorbital area remarkably broad, lacking deep
+constriction; frontals between orbits greatly inflated laterally,
+postorbital prominence inconspicuous; mesopterygoid fossa extending to
+level of posterior margin of M3; I having sulcus broader than in other
+subgenera, mostly on inner half of anterior surface but sometimes
+overlapping mid-line; enamel plate lacking from posterior wall of P4,
+rarely retaining narrow vestige near lingual border of posterior loph;
+M3 having distinct heel, bicolumnar pattern with inner re-entrant fold
+usually minute, occlusal length less than in other subgenera, length
+less than combined lengths of M1-2; hair generally coarse, sometimes
+hispid, sparse, in lowland forms, so sparse as to impart appearance of
+nakedness.
+
+_Referred species and subspecies._--Fourteen taxa:
+
+    _Orthogeomys grandis alleni_ Nelson and Goldman, 1930. Jour. Mamm.,
+           11:156, May 9. Type from near Acapulco, 2000 ft., Guerrero.
+
+    _Orthogeomys grandis annexus_ Nelson and Goldman, 1933. Proc. Biol.
+           Soc. Washington, 46:195, October 26. Type from Tuxtla
+           Gutierrez, 2600 ft., Chiapas.
+
+    _Orthogeomys grandis carbo_ Goodwin, 1956. Amer. Mus. Novit.,
+           1757:5, March 8. Type from Excurano, 2500 ft., Cerro de San
+           Pedro, 20 km. W Mixtequilla, Oaxaca.
+
+    _Orthogeomys grandis felipensis_ Nelson and Goldman, 1930. Jour.
+           Mamm., 11:157, May 9. Type from Cerro San Felipe, 10 mi. N
+           Oaxaca, Oaxaca.
+
+    _Orthogeomys grandis huixtlae_ Villa, 1944. Anal. Inst. Biol. Univ.
+           Nac. México, 15:319. Type from Finca Lubeca, 12 km. NE
+           Huixtla, 850 m., Chiapas.
+
+    _Orthogeomys grandis grandis_ (Thomas, 1893). Ann. Mag. Nat. Hist.,
+           ser. 6, 12:270, October. Type from Dueñas, Guatemala.
+
+    _Orthogeomys grandis latifrons_ Merriam, 1895. N. Amer. Fauna,
+           8:178, January 31. Type from Guatemala, exact locality
+           unknown.
+
+    _Orthogeomys grandis nelsoni_ Merriam, 1895. N. Amer. Fauna, 8:176,
+           January 31. Type from Mt. Zempoaltepec, 8000 ft., Oaxaca.
+
+    _Orthogeomys grandis pluto_ Lawrence, 1933. Proc. New England Zool.
+           Club, 13:66, May 8. Type from Cerro Cantoral, north of
+           Tegucigalpa, Honduras.
+
+    _Orthogeomys grandis scalops_ (Thomas, 1894). Ann. Mag. Nat. Hist.,
+           ser. 6, 13:437, May. Type from Tehuantepec, Oaxaca.
+
+    _Orthogeomys grandis soconuscensis_ Villa, 1949. Anal. Inst. Biol.
+           Univ. Nac. México, 19:267, April 8. Type from Finca
+           Experanza, 710 m., 45 km. (by road) NW Huixtla, Chiapas.
+
+    _Orthogeomys grandis guerrerensis_ Nelson and Goldman, 1930. Jour.
+           Mamm., 11:158, May 9. Type from El Limón, in valley of Río de
+           las Balsas approximately 20 mi. NW La Unión, Guerrero.
+
+    _Orthogeomys cuniculus_ Elliot, 1905. Proc. Biol. Soc. Washington,
+           18:234, December 9. Type from Zanatepec, Oaxaca.
+
+    _Orthogeomys pygacanthus_ Dickey, 1928. Proc. Biol. Soc. Washington,
+           41:9, February 1. Type from Cacaguatique, 3500 ft., Dept. San
+           Miguel, El Salvador.
+
+
+Subgenus =Heterogeomys= Merriam
+
+    1895. _Heterogeomys_ Merriam, N. Amer. Fauna, 8:179, January 21.
+
+_Type._--_Geomys hispidus_ Le Conte, 1852, from near Jalapa, Veracruz.
+
+_Chronologic range._--Late Pleistocene, Wisconsin deposits (San
+Josecito Cave local fauna, Nuevo León) to the Recent.
+
+_Description._--Skull dolichocephalic (less so than in the other
+subgenera); zygomata more widely spreading than in _Orthogeomys_;
+ramus and angular process short; interorbital area noticeably
+constricted; frontals between orbits neither exceptionally broad or
+inflated; mesopterygoid fossa extending to level of posterior margin
+of M3; I having sulcus on inner third of anterior surface usually
+narrower than in subgenus _Orthogeomys_; enamel plate on posterior
+wall of P4 restricted to lingual half of loph; M3 distinctly
+biprismatic, posterior loph usually circumscribed by shallow inner
+re-entrant fold and outer deep fold well developed in all members of
+genus; posterior loph forming conspicuous heel longer than in subgenus
+_Orthogeomys_; occlusal length equal to or slightly less than combined
+lengths of M1-2; hair coarse and hispid but never so sparse as to
+impart appearance of nakedness.
+
+_Referred species and subspecies._--Eleven taxa:
+
+    *_Orthogeomys onerosus_ (Russell, 1960). Univ. Kansas Publ., Mus.
+        Nat. Hist., 9 (21):544, January 14. Type from San Josecito Cave
+        local fauna, Upper Pleistocene, Nuevo León.
+
+    _Orthogeomys hispidus cayoensis_ (Burt, 1937). Occ. Papers Mus.
+        Zool., Univ. Michigan, 365:1, December 16. Type from Mountain
+        Pine Ridge, 12 mi. S El Cayo, British Honduras.
+
+    _Orthogeomys hispidus chiapensis_ (Nelson and Goldman, 1929). Proc.
+        Bio. Soc. Washington, 42:151, March 30. Type from Tenejapa, 16
+        mi. NE San Cristobal, Chiapas.
+
+    _Orthogeomys hispidus concavas_ (Nelson and Goldman, 1929). Proc.
+        Biol. Soc. Washington, 42:148, March 30. Type from Pinal de
+        Amoles, Querétaro.
+
+    _Orthogeomys hispidus hispidus_ (Le Conte, 1852). Proc. Acad. Nat.
+        Sci. Philadelphia, 6:158. Type from near Jalapa, Veracruz.
+
+    _Orthogeomys hispidus latirostris_ (Hall and Alvarez, 1961). Anal.
+        Escuela Nac. Ciencias Biol., 10:121, December 20. Type from
+        Hacienda Tamiahua, Cabo Rojo, Veracruz.
+
+    _Orthogeomys hispidus negatus_ (Goodwin, 1953). Amer. Mus. Novit.,
+        1620:1, May 4. Type from Gomez Ferias, 1300 ft., about 45 mi. S
+        Ciudad Victoria, 10 km. W Pan American Highway, Tamaulipas.
+
+    _Orthogeomys hispidus tehuantepecus_ (Goldman, 1939). Jour.
+        Washington Acad. Sci., 29:174, April 15. Type from mountains 12
+        mi. NW Santo Domingo and about 60 mi. N Tehuantepec, 1600 ft.,
+        Oaxaca.
+
+    _Orthogeomys hispidus torridas_ (Merriam, 1895). N. Amer. Fauna,
+        8:183, January 31. Type from Chichicaxtle, Veracruz.
+
+    _Orthogeomys hispidus yucatanensis_ (Nelson and Goldman, 1929).
+        Proc. Biol. Soc. Washington, 42:150, March 30. Type from
+        Campeche, Campeche.
+
+    _Orthogeomys lanius_ (Elliot, 1905). Proc. Biol. Soc. Washington,
+        18:235, December 9. Type from Xuchil, Veracruz.
+
+
+Subgenus =Macrogeomys= Merriam
+
+    1895. _Macrogeomys_ Merriam, N. Amer. Fauna, 8:185, January 31.
+
+_Type._--_Geomys heterodus_ Peters, 1865, from Costa Rica, exact
+locality unknown.
+
+_Chronologic range._--Known only from the Recent.
+
+_Description._--Skull dolichocephalic in varying degree (overlapping
+subgenera _Orthogeomys_ and _Heterogeomys_ in this respect); mandibles
+elongated, not spreading far laterally; angular processes decidedly
+short; breadth across zygomata in no instance significantly exceeding
+mastoid breadth; interorbital area strongly constricted; frontals
+between orbits slightly inflated laterally (especially in forms having
+more strongly dolichocephalic skulls); postorbital prominence
+conspicuous; anterior margin of mesopterygoid fossa terminating well
+behind M3; I having narrow and deep sulcus entirely on inner third of
+anterior surface; enamel plate on posterior wall of P4 restricted to
+inner half of loph; M3 bilophodont (outer and inner re-entrant folds
+each circumscribing a loph), posterior loph remarkably elongated and
+forming pronounced heel, length of crown more than combined lengths of
+M1-2; hair wooly in some individuals, harsh in others but seldom
+hispid, never so sparse as in subgenus _Orthogeomys_; some species
+having white markings, especially on lumbar region and head.
+
+_Referred species and subspecies._--Eleven taxa:
+
+    _Orthogeomys heterodus cartagoensis_ (Goodwin, 1943). Amer. Mus.
+        Novit., 1227:2, April 22. Type from Paso Ancho, Province
+        Cartago, Costa Rica.
+
+    _Orthogeomys heterodus dolichocephalus_ (Merriam, 1895). N. Amer.
+        Fauna, 8:189, January 31. Type from San José, Costa Rica.
+
+    _Orthogeomys heterodus heterodus_ (Peters, 1865). Monatsb. preuss.
+        Acad. Wiss., Berlin, 1865:177. Type from Costa Rica, exact
+        locality unknown.
+
+    _Orthogeomys cavator nigrescens_ (Goodwin, 1943). Amer. Mus. Novit.,
+        1227:3, April 22. Type from El Muneco (Río Navarro), 10 mi. S
+        Cartago, 4000 ft., Province Cartago, Costa Rica.
+
+    _Orthogeomys cavator pansa_ (Bangs, 1902). Bull. Mus. Comp. Zool.,
+        39:44, April. Type from Bogava (= Bugaba), 600 ft., Chiriquí,
+        Panamá.
+
+    _Orthogeomys dariensis_ (Goldman, 1912). Smithsonian Misc. Coll.,
+        60(2):8, September 20. Type from Cana, 2000 ft., mountains of
+        eastern Panamá.
+
+    _Orthogeomys underwoodi_ (Osgood, 1931). Field Mus. Nat. Hist.,
+        Publ. 295, Zool. Ser., 185:143, Aug. 3. Type from Alto de
+        Jabillo Pirris, between San Geronimo and Pozo Azul, western
+        Costa Rica.
+
+    _Orthogeomys cherriei carlosensis_ (Goodwin, 1943). Amer. Mus.
+        Novit., 1227:3, April 22. Type from Cataratos, San Carlos,
+        Alajuela, Costa Rica.
+
+    _Orthogeomys cherriei cherriei_ (J. A. Allen, 1893). Bull. Amer.
+        Mus. Nat. Hist., 5:337, December 16. Type from Santa Clara,
+        Costa Rica.
+
+    _Orthogeomys cherriei costaricensis_ (Merriam, 1895). N. Amer.
+        Fauna, 8:192, January 31. Type from Pacuare, Costa Rica.
+
+    _Orthogeomys matagalpae_ (J. A. Allen, 1910). Bull. Amer. Mus. Nat.
+        Hist., 28:97, April 30. Type from Peña Blanca, Matagalpa,
+        Nicaragua.
+
+
+Genus =Pappogeomys= Merriam
+
+    1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31.
+
+    1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31.
+          Type: _Geomys merriami_ Thomas.
+
+    1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31.
+          Type: _Geomys gymnurus_ Merriam; Hooper, Jour. Mamm.,
+          27:397, November 25, 1946.
+
+_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope
+Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.
+
+_Chronologic range._--Late Pliocene, from deposits of early Blancan
+age (Benson local fauna, Arizona) to the Recent. However in the
+Pleistocene, only late Pleistocene records are known, and
+_Pappogeomys_ has not been found in early (late Blancan) or middle
+(Irvingtonian) Pleistocene local faunas. Presumably the genus was
+restricted to México during the Pleistocene until post-Wisconsin time.
+
+_Description and discussion._--The size ranges from as little as in
+the smaller kinds of _Thomomys_ to the maximum attained in the
+subfamily and matched elsewhere perhaps in only a few of the larger
+subspecies of _Orthogeomys grandis_. Depending on the species and
+subgenus, the form of the skull varies from generalized to
+specialized. The generalized skulls are short and not especially
+narrow; the zygomatic arches are spread laterally so far that the
+breadth across them exceeds the breadth across the mastoid processes.
+The most specialized skulls are platycephalic and the breadth across
+the mastoid processes equals or exceeds the breadth across the
+zygomatic arches (even so, the zygomatic arches are still relatively
+widespread). In correlation with the great breadth of the posterior
+part of the cranium, the rami of the mandibles diverge widely
+posteriolaterally and the angular processes are remarkably elongated.
+The rostrum is moderately broad in most species, but not nearly so
+broad and heavy as in _Orthogeomys_.
+
+The single deep, median sulcus on the outer surface of the upper
+incisor is slightly displaced to the inner side of the tooth. The
+posterior surface of P4 lacks enamel (small vestige found on lingual
+end of posterior wall in only two adult individuals--UA 3260 and KU
+100442, of the subgenus _Pappogeomys_); the other three plates are
+fully developed as usual. The p4 is provided with four fully developed
+enamel plates, in the pattern characteristic of the tribe Geomyini. In
+the p4 of the late Pliocene species (_P. bensoni_) the re-entrant
+angles are open (obtuse), a trait that is evidently primitive in the
+Geomyini.
+
+All three lower molars are single, compressed, elliptical columns with
+enamel on only the posterior surfaces. M1 and M2 are also elliptical
+in cross-section and decidedly anteroposteriorly compressed, like the
+lower molars. Nevertheless, the enamel pattern is variable; enamel
+plates may be retained completely across both the anterior and
+posterior walls of M1 and M2 or only the anterior plate may be
+retained without reduction and the posterior plate may be reduced so
+that only a vestige is retained on the lingual fourth of the tooth or
+the posterior plate may be completely lost.
+
+M3 tends to remain at least incompletely bilophodont by reason of
+retaining a permanent labial re-entrant fold in most species (with
+exceptions in _Pappogeomys bulleri_ and some old adults of _P.
+castanops_). Primitively the occlusal surface of M3 is subtriangular
+(subgenus _Pappogeomys_), but in the _castanops_ species-group of the
+advanced subgenus _Cratogeomys_, the posterior loph usually is reduced
+and the occlusal surface is quadriform or obcordate. Curiously, the
+trend towards reduction of the posterior loph is reversed in one
+subspecies (_P. merriami fulvescens_) and, the loph has elongated into
+a pronounced heel in some specimens, resembling the condition in
+_Orthogeomys_. The entire range of variation occurs in _P. m.
+fulvescens_. The subtriangular pattern is retained in the most
+specialized species of _Cratogeomys_ where that pattern is associated
+with extreme platycephaly in the _gymnurus_ species-group. In most
+species the posterior loph supports two lateral plates, the outer one
+always bordering the labial re-entrant fold. In _Pappogeomys bulleri_
+and in the _castanops_ species-group, the outer re-entrant fold of M3
+tends to be obsolete, and the tooth becomes quadriform or
+suborbiculate in some individuals and loses the bilophodont pattern
+that characterizes other species. The lingual enamel plate is
+displaced to the posterior surface of the tooth, and one or both
+plates may disappear with advancing age. Consequently, only the
+anterior enamel plate remains in some adults, and constitutes the
+maximum degree of reduction of enamel on M3 in the Geomyinae. In many
+adults of _Pappogeomys bulleri_, the enamel investment of the
+posterior loph is complete and the two lateral plates are connected,
+without interruption around the posterior apex of the tooth, evidently
+representing the retention of a primitive character of the ancestral
+lineage.
+
+The m3 of _P. bensoni_ from the late Pliocene is distinguished by
+minute lateral inflections suggesting the primitive biprismatic
+pattern. Also the posterior enamel plates of m1 and m2 are remarkably
+long, extending around the ends of the tooth. The associated upper
+incisor was unisulcate as in the modern species, and the basitemporal
+fossa of the mandible is well developed and deep.
+
+The lower jaw is stout and relatively short. The masseteric ridge is
+well developed and has an especially thick crest. The basitemporal
+fossa is deep. In most living species, the pelage is soft and dense,
+but in one species, _Pappogeomys fumosus_, the hairs are coarse and
+hispid somewhat as in _Orthogeomys_.
+
+
+Key to the Subgenera of _Pappogeomys_
+
+  A  Enamel plates completely developed across posterior walls of
+     M1 and M2, except in one species (_P. alcorni_) having enamel
+     restricted to lingual fourth in M1; sagittal crest lacking
+     owing to impressions of temporal muscles remaining separated
+     (even in old adults); zygomata slender, and without platelike
+     expansion at lateral angle. Subgenus _Pappogeomys_         p. 534
+
+  A´ Enamel lacking on posterior walls of M1 and M2; pronounced
+     sagittal crest developed in adults of both sexes by union
+     of temporal impressions at middorsal line; zygomata stout
+     and wide, with lateral angle expanded into broad plate.
+     Subgenus _Cratogeomys_                                     p. 535
+
+
+Subgenus =Pappogeomys= Merriam
+
+    1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31.
+
+_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope
+Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.
+
+_Chronologic range._--Late Pliocene (Benson local fauna, Arizona) to
+Recent, but no specimens known from Pleistocene.
+
+_Description._--Small, approximately same size as small subspecies of
+_Thomomys umbrinus_ but forefeet larger and claws longer; skull of
+generalized shape, broad, relatively short, smoothly rounded, not
+especially compressed dorso-ventrally; zygomatic breadth great but not
+exceeding mastoid breadth; zygomata relatively slender for geomyid and
+lacking platelike expansions at lateral angles; rostrum relatively
+narrow; sagittal crest lacking, owing to impressions of temporal
+muscles remaining separated; angular process of mandible not
+especially elongated; enamel plates extending completely across
+posterior wall of M1 and M2, except in one species, _P. alcorni_,
+where posterior plate of M1 remains only on lingual fourth of
+posterior wall (remainder of plate lacking); with wear, plates
+sometimes exceptionally thin completely across posterior face of M2
+and especially M1 in a few individuals of _P. bulleri_ much as Paulson
+(1961:138-139) describes in extinct _Geomys tobinensis_; one or both
+plates rarely disappear in final stages of attrition in old
+individuals resulting in same dental pattern found in _Cratogeomys_;
+M1 and M2 retaining enamel plate on anterior wall throughout life; M3
+usually subtriangular in cross-section but sometimes suborbiculate or
+ovoid, crown slightly bilophodont owing to shallowness of labial
+re-entrant angle in modern species; posterior loph of M3 not
+especially elongated and crown not significantly longer than wide;
+both lateral enamel plates of M3 usually well developed and
+approximately equal in length, occasionally plates reduced in length
+and rarely one or both plates are lost with wear in old individuals;
+patch of whitish or buffy hairs surrounding nose of most individuals.
+
+The primitive character of the lower dentition, as described in the
+species account above, suggest that _Cratogeomys_ [= _Pappogeomys_]
+_bensoni_ Gidley should be referred to the subgenus _Pappogeomys_
+rather than _Cratogeomys_. Only the upper dentition would make
+positive identification possible; however, reference to the subgenus
+_Pappogeomys_ seems to be the best arrangement at this time.
+
+_Referred species._--Three (one extinct):
+
+    *_Pappogeomys bensoni_ (Gidley), 1922. U. S. Geol. Surv. Prof.
+        Papers, 131:123. Type from Benson local fauna (late Pliocene),
+        Cochise County, Arizona.
+
+    _Pappogeomys alcorni_ Russell, 1957. Univ. Kansas Publ. Mus. Nat.
+        Hist., 9(11):359. Type from 4 mi. W Mazamitla, Jalisco.
+
+    _Pappogeomys bulleri_ Thomas, 1892. Ann. Mag. Nat. Hist., Ser. 6,
+        vol. 10:196, August. Type from "near Talpa," west slope of
+        Sierra Madre de Mascota, Jalisco.
+
+
+Subgenus =Cratogeomys= Merriam
+
+    1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31.
+
+    1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31.
+          Type: _Geomys gymnurus_ Merriam, 1892.
+
+_Type._--_Geomys merriami_ Thomas, 1893, from "Southern México,"
+probably in Valley of México.
+
+_Chronologic range._--Late Pleistocene, from Wisconsin deposits (San
+Josecito Cave, Nuevo León, Upper Bercerra, México, and Burnet Cave,
+New Mexico, local faunas) to the Recent.
+
+_Description._--Size medium to large; skull becoming angular and
+rugose with age, and tending towards platycephaly and dorso-ventral
+compression; zygomata stout, each bearing platelike expansion at
+anterolateral angle into which anterior end of jugal becomes morticed;
+breadth across zygomata great relative to length of skull; rostrum
+relatively broad; squamosals expanding medially with age eventually
+growing over lateral parts of parietals, and sometimes also expanding
+laterally displacing postglenoid notch; sagittal crest well developed
+in adults of both sexes, but especially high and bladelike in males;
+lambdoidal crest prominent in all but young animals, having dorsal
+outline broadly convex posteriorly in most species but strongly
+sinuous in _gymnurus_-group; enamel plate on posterior wall of P4
+absent; enamel plates present only on anterior walls of M1 and M2; M3
+variform in occlusal shape (as described in species account), either
+subtriangular (_gymnurus_-group), quadriform or obcordate
+(_castanops_-group, with exceptions as noted before); lateral plates
+of M3 usually present in all species, labial plate approximately as
+long as lingual plate in _gymnurus_-group (like that in subgenus
+_Pappogeomys_) or distinctly shorter in _castanops_-group (labial
+plate scarcely extending beyond border of labial re-entrant fold); one
+or both lateral plates tending to disappear with wear in
+_castanops_-group, with lingual plate usually disappearing first;
+breadth across angular processes clearly more than breadth across
+zygomatic processes, especially in _gymnurus_-group.
+
+_Remarks._--In the species of the _castanops_-group the skulls can be
+spoken of as generalized and the least platycephalic of the subgenus.
+Indeed, the species of the _castanops_-group are hardly more
+specialized in this respect than is the subgenus _Pappogeomys_.
+In these skulls the breadth across the squamosal processes is less
+than that across the zygomatic arches, although the two dimensions
+are almost equal in some examples of _P. merriami_ of the
+_castanops_-group (where squamosal breadth varies from 85 to 98% of
+zygomatic breadth). In the species having marked platycephalic skulls
+(_gymnurus_ species-group) the breadth across the squamosal processes
+equals or exceeds the breadth across the zygomatic arches (squamosal
+breadth rarely 97 to 99% of zygomatic breadth), except in _P. zinseri_
+and _P. tylorhinus zodius_.
+
+The variable character of the third upper molar as between species
+suggests that this tooth is presently undergoing active evolution. The
+structure of this tooth, although differing between taxa, is
+remarkably stable in other kinds of Geomyini. The most remarkable
+modification of M3 in _Cratogeomys_ is the obcordate pattern
+developed in _P. merriami_ of the _castanops_-group. The posterior
+loph and entire tooth is shortened somewhat resembling in shape that
+of _Thomomys_. Moreover, the posterior loph is twisted labially;
+consequently, its posterior surface now forms the labial border of the
+weakly defined posterior loph. Owing to the torsion, the lingual
+enamel plate has been rotated to the posterior surface of the tooth.
+Therefore, the tooth is provided with two transverse enamel plates,
+including the plate on the anterior wall of the tooth. The labial
+plate is greatly reduced, its total surface being restricted to the
+small labial inflection. The highly specialized obcordate M3 is not
+found in the most specialized platycephalic skulls characteristic of
+the _gymnurus_ species-group. Instead the _gymnurus_-group retains the
+primitive subtriangular pattern without significant modification.
+
+_Referred species._--Seven:
+
+
+_castanops_ species-group
+
+    _Pappogeomys castanops_ (Baird, 1852). Report Stanbury's Exp'd. to
+        Great Salt Lake, p. 313, June. Type from "Prairie road to Bent's
+        Fort," near present town of Las Animas, Colorado.
+
+    _Pappogeomys merriami_ (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6,
+        12:271, October. Type from "southern Mexico," probably Valley of
+        México (see Merriam, 1895:152).
+
+
+_gymnurus_ species-group
+
+    _Pappogeomys fumosus_ (Merriam, 1892). Proc. Biol. Soc. Washington,
+        7:165, September 29. Type from 3 mi. W Colima, Colima.
+
+    _Pappogeomys gymnurus_ (Merriam, 1892). Proc. Biol. Soc. Washington,
+        7:166, September 29. Type from Zapotlan (Ciudad Guzman),
+        Jalisco.
+
+    _Pappogeomys neglectus_ (Merriam, 1902). Proc. Biol. Soc.
+        Washington, 15:68, March 22. Type from Cerro de la Calentura,
+        about 8 mi. NW Pinal de Amoles, Querétaro.
+
+    _Pappogeomys tylorhinus_ (Merriam, 1895). N. Amer. Fauna, 8:167,
+        January 31. Type from Tula, Hidalgo.
+
+    _Pappogeomys zinseri_ (Goldman, 1939). Jour. Mamm., 20:91, February
+        15. Type from Lagos, Jalisco.
+
+
+
+
+PHYLOGENY OF THE GEOMYIDAE
+
+
+The fossil record of the Geomyidae provides a sequence of morphotypes,
+each representing a stage in the phyletic development of the family.
+Most of the preserved specimens probably represent the stufenreihe
+rather than the ahnenreihe, as Simpson (1953:219-220) points out. Even
+so, the stufenreihe closely approximates the general trend of
+evolution, and the level of structural organization in the different
+stages of phyletic development may be ascertained. The actual
+ancestral series of most lineages probably will remain unknown, but
+hopefully some of the existing gaps will be filled by future
+discoveries. From the established record, several clearly defined
+lineages can be distinguished; in fact the sequence of origin, pattern
+of evolution, and specializations, of the principal lineages are
+reasonably well expressed.
+
+
+Primitive Morphotype
+
+In the earliest known geomyids from the Upper Oligocene and Lower
+Miocene, the premolars and molars are biprismatic and bilophodont. In
+rodents, this is itself a specialized pattern, and is thought to have
+evolved from a more primitive sextituberculate prototype by the union
+of individual cusps, and probably also cuspules, forming the two
+transverse enamel lophs. The primitive, common ancestor of the
+Geomyidae and Heteromyidae with sextituberculate teeth in the early
+Tertiary is unknown.
+
+As soon as geomyids attained the early bilophodont stage of evolution,
+the basic morphological structure of the family was established. The
+family probably first became clearly distinguished from other
+Geomyoidea at this stage. In the early bilophodont stages of
+evolution, owing to the relatively deep valley between them, the two
+columns probably failed to unite in the normal cycle of wear, as they
+do in all later geomyids. _Griphomys_ described by Wilson (1940:93)
+from the late Eocene of California, has a bilophate pattern in which
+the anterior and posterior lophs are separated by a persistent
+transverse valley. The occlusal pattern of _Griphomys_ closely
+resembles a stage through which the ancestors of the early Miocene
+geomyids must have passed in their pre-Miocene evolution, as Wilson
+suggests (1949:115-116). Although he (1940:95; 1949:110-118)
+tentatively referred _Griphomys_ to the superfamily Geomyoidea and
+Simpson (1945:80) went so far as to refer it to the family Geomyidae,
+with a notation of _incertae sedis_, its exact relationship to the
+pocket gophers is uncertain. However, the structure of the molariform
+dentition of _Griphomys_ does not exclude it from the phyletic
+ancestry of the Geomyidae. In subsequent stages of evolution the
+anterior and posterior columns become united. Thereby part of the
+valley floor between the transverse prisms was progressively elevated,
+to the stage where attrition on the occlusal surface would unite the
+two columns. On the unworn enamel cap of living geomyids the two
+transverse enamel folds are separated by a shallow but well defined
+valley, briefly reflecting the ancient ancestral pattern.
+
+Union of the lophs may have been either at the mid-points of the two
+columns or at the edge of their protomeres. [A protomere is the half
+of a tooth containing the protocone or protoconid--lingual side of
+upper tooth and labial side of lower tooth. The paramere is the
+opposite half of a given tooth--labial side of upper tooth and
+lingual side in lower tooth. See Miller and Gidley, 1918:434.] Union
+of the columns at the mid-points would have produced the figure-8
+occlusal pattern (or H-pattern), which is characteristic of the early
+Miocene Geomyinae (_Dikkomys_). Union of the two columns at the
+protomeres would have produced the U-shaped pattern of the
+Entoptychinae, which also occurred in the early Miocene and were
+contemporary with the earliest Geomyinae. Since pre-Miocene geomyids
+are unknown, the actual phyletic development of the dentition is a
+matter of speculation. Probably the development of the two divergent
+lineages, one leading to the Entoptychinae and the other to the
+subfamily Geomyinae, occurred in the Oligocene (as depicted in Fig.
+3). Of the two lineages, the subfamily Geomyinae, in my view, is the
+more primitive and less specialized. Support for this view is
+furnished by a reconstruction of the pattern of occlusal wear in
+_Dikkomys_ and _Pliosaccomys_, especially on the first and second
+molars.
+
+In _Dikkomys_, the anterior and posterior column first unite near
+their mid-points in the first stages of wear thus producing a figure-8
+shaped (H-shaped) occlusal pattern in the premolar and all three
+molars. Evidently in the first two upper molars, the columns unite
+closer to their lingual margins than their mid-points, but at any rate
+both outer and inner re-entrant folds are evident at this stage of
+wear. With continued attrition on m1 and m2 of _Dikkomys_, the
+anterior and posterior columns secondarily unite at the edge of their
+labial margins thus enclosing a fossette of enamel in the labial half
+of the tooth. The lateral coalescence at the ends of the protomeres
+occurs because of the shallow vertical depth of the labial re-entrant
+fold, and the fossette itself does not reach the base of the crown and
+with continued wear it too would disappear, but not until the last
+stages of wear, at least in _Dikkomys matthewi_. The lingual
+re-entrant fold is deep, and therefore, persistent through all stages
+of wear. Although the amount of wear required for its effacement would
+be great, the occlusal configuration of the first and second lower
+molars in _Dikkomys_ could be eventually ground down to a U-pattern as
+in the entoptychids. Only one upper molar of _Dikkomys_, the first,
+has been recovered (see Wood, 1936:23, fig. 32B). Although the tooth
+is in an early stage of wear, the lingual valley is minute. Less
+attrition than required in m1 and m2 would progressively reduce the
+lingual fold until it too would essentially form a U-pattern, perhaps
+retaining a slight lingual inflection. Hence, the first upper molar
+becomes a mirror image of the first lower molar, and the second upper
+molar probably had the same pattern as the first (at least it does so
+in _Pliosaccomys_). Both of the lateral re-entrant folds of the
+premolar are deep vertically, and consequently would not disappear
+with occlusal wear. Therefore, the H-pattern of the premolars is
+retained throughout life.
+
+The m3 (M3 unknown for _Dikkomys_ or _Pliosaccomys_) also has deep
+lateral folds; hence, it too retains the H-pattern in all stages of
+attrition, although the isthmus between the two prisms may become
+wider in the final phases of wear (as it does in _Pliosaccomys_).
+
+In _Pliosaccomys_, the stages of wear are essentially the same as
+those described for _Dikkomys_, except that the anterior and posterior
+loph of the first and second molars tend to unite closer to one side
+of the tooth, lingual side in upper molars and labial in lower. Only a
+slight inflection of the re-entrant fold is evident on the side of
+union, and the inflection disappears in the first phases of wear as
+the columns unite. Concomitant with the lateral shift in the initial
+point of coalescence of the transverse lophs, the occlusal penetration
+of the re-entrant fold from the opposite side increases in horizontal
+depth, and the fold extends medially more than half way across the
+occlusal surface, thus forming a pattern essentially like that of the
+entoptychids. The U-pattern in _Pliosaccomys_ appears in the initial
+stages of wear without going through an earlier H-pattern as is the
+case in its Miocene ancestors of the genus _Dikkomys_, unless the
+minute inflection is considered as indicative of that stage. The two
+columns of the premolar and m3 are joined near their mid-points as in
+_Dikkomys_; therefore, they retain their primitive H-pattern, a
+feature unique to the Geomyinae.
+
+The evolutionary trend toward an ontogenetically earlier U-pattern in
+the first two molars in the primitive lineage of the Geomyinae
+suggests that the U-pattern characteristic of the Entoptychinae was
+simply an earlier tendency toward the same specialization that
+occurred later in the subfamily Geomyinae. If so, early entoptychines
+would have been characterized by an H-pattern in the first stages of
+attrition, like _Dikkomys_, and later developed union at the edge of
+the protomeres. However, in the entoptychines, all the molariform
+dentition, and not merely the first and second molar, became
+specialized; consequently the U-pattern was produced on the occlusal
+surfaces of each of the cheek teeth. As in _Pliosaccomys_, the
+transitional phase, in which the two columns were united at their
+mid-points, was eventually eliminated from the pattern of wear and
+only the U-pattern, that now appeared in the initial stages of wear,
+was retained. In the entoptychines of the early Miocene there is no
+suggestion of the H-pattern that characterizes the Geomyinae, except
+in the position of the cusps before wear in the lower molars of
+_Pleurolicus sulcifrons_, which, according to Wood (1936:6), suggests
+the H-pattern. In earlier unknown Oligocene stages of evolution, the
+prisms possibly united first at their mid-points, and the columns may
+have joined at the side of the tooth only in the terminal stages of
+wear. The U-pattern of pre-Miocene entoptychines, therefore, may have
+become the dominant occlusal pattern only in the later stages of
+phyletic development.
+
+According to the recently expressed views of several paleontologists,
+the Entoptychinae constitute the primitive lineage of the family and
+the early Geomyinae constitute a specialized offshoot of the
+entoptychine ancestral assemblage. The structure of the Entoptychinae,
+especially of the less advanced genera, closely approximates that of
+the hypothetical primitive morphotype. But, according to my view, the
+subfamily Geomyinae constitutes the ancestral assemblage and its
+structure is essentially that of the primitive morphotype of the
+family. At any rate the structure of the early geomyines more closely
+approximates the structure of the ancestral stock than the more
+divergent entoptychines. Therefore, the genus _Dikkomys_ of the early
+Miocene, the first known geomyine, is considered to be a generalized
+geomyid, and, although it is a contemporary of the more specialized
+entoptychid assemblage, is considered to be more closely allied to the
+ancestral stock.
+
+The entoptychines were the dominant and most highly differentiated
+geomyids of the early and middle Miocene. Nevertheless, they became
+extinct in the middle Miocene, and the geomyines of that time survived
+and later gave rise to the modern pocket gophers. Therefore, the early
+history of the family Geomyidae is characterized by an early radiation
+and trend toward specialization, followed by survival of the less
+specialized Geomyinae and extinction of the more specialized
+Entoptychinae.
+
+
+Entoptychid Radiation
+
+The most abundant geomyids of the early and middle Miocene, the
+Entoptychinae, consisted of at least 24 species (see Wood, 1936:4-25)
+classified in four genera: _Pleurolicus_, _Gregorymys_, _Grangerimus_,
+and _Entoptychus_. The genera were essentially contemporaneous (see
+Figure 3). Even so, the subfamily was morphologically varied, pointing
+to an earlier origin in the Oligocene (actually a part of the John Day
+Fauna, including _Pleurolicus_ may be correlated with late Oligocene
+Whitneyian age) followed by a relatively rapid radiation including all
+four genera in the early Miocene. Two genera, _Pleurolicus_ and
+_Gregorymys_, continued into the Middle Miocene (Hemingfordian). This
+divergence, specialization, and subsequent radiation suggest that the
+entoptychines evolved into a new major adaptive zone, in the sense
+described by Simpson (1945:199-206).
+
+The radiation is correlated geographically and temporally with the
+southward retreat of the Neotropical flora of the Tertiary from the
+western United States and southward movement of the Arctic flora of
+the Tertiary (see Axlerod, 1950; Berry, 1937:31-46; Chaney,
+1947:139-148; and Kendeigh, 1961:280-283). In the early Tertiary the
+Neotropical-tertiary geoflora occurred northward to at least 49°
+latitude in western North America, and the boreal Arctic-tertiary
+flora was restricted to a circumpolar zone. The southward and eastward
+shift of the Neotropical-tertiary flora, associated with the drying
+and chilling of the continent, began in the middle or late Oligocene
+and was concurrent with the divergence and radiation of the
+Entoptychinae. Beginning in late Oligocene and continuing at least
+into middle Miocene, most of the region in which the entoptychines
+occurred was occupied by the Arcto-tertiary geoflora of which the
+temperate forest division contributed the dominate plant associations.
+The maples, chestnuts, dogwoods, beeches, walnuts, oaks, elms,
+birches, and sycamores of that flora were the forerunners of today's
+eastern deciduous forest. It is my view that the entoptychines became
+adapted to the conditions of this paleoecological environment and
+radiated rapidly in the Arikareean when the major change occurred in
+climax vegetation. The ancestral stock of the Geomyinae was not so
+successful in the Arcto-tertiary climax, and most of it probably was
+displaced southward along with the tropical flora.
+
+The skeleton in the entoptychines is not so strongly fossorial as in
+the modern geomyids (Wilson, 1949:117), and these early geomyids
+probably were semi-fossorial with somewhat the same burrowing habits
+as those of the living mountain beaver (_Aplodontia_). Inasmuch as the
+morphology and taxonomy of the entoptychines were discussed in detail
+by Cope (1884) and reviewed later by Wood (_loc. cit._), there is no
+need to recount the details here. According to Wood (_op. cit._,
+27-28), _Pleurolicus_ occupied a central position in the entoptychid
+radiation and perhaps appeared slightly earlier than the other genera.
+Wilson (1949) suggested that the lower part of the John Day may
+actually be Upper Oligocene rather than Lower Miocene, and this
+arrangement is followed here. Also, _Pleurolicus_ is less specialized
+than the other genera and occurs in deposits of both the Great Plains
+and the Pacific Coast. _Gregorymys_, also little specialized, occurred
+only on the Great Plains. The more specialized genera, _Grangerimus_
+and _Entoptycus_, evidently appeared somewhat later than _Pleurolicus_
+and evolved from it. Except for a record from southern Texas reported
+recently by Hibbard and Wilson (1950:621-623) and the new species
+described by MacDonald (1963:182) from the Sharps Formation of South
+Dakota (early Arikareean), _Grangerimus_ is known only from the
+Pacific coast. _Entoptycus_ was restricted to the Pacific Coast (John
+Day fauna).
+
+_Entoptycus_ is the most specialized of the known genera; it has
+pronounced fossorial adaptations, especially in the skull. Its
+molariform teeth are rootless and ever-growing as in the modern
+geomyines. Moreover, the continuous enamel bands on only moderately
+worn teeth become separated in the final stages of wear into anterior
+and posterior enamel plates by tracts of dentine that extend toward
+the crown on the sides of each tooth. This extension was made possible
+by the union of the two columns at both the lingual and labial margins
+of the tooth forming an O-pattern, and the crown is essentially
+monoprismatic save for the isolated enamel fossette in the center of
+the tooth. The fossette is all that remains of the lateral re-entrant
+fold that characterized the preceding U-pattern of the earlier stages
+of wear. Late in the sequence of wear, the anterior enamel plate is
+lost in the lower molars and the posterior plate in the upper molars.
+The U-pattern characterizes the final stages of attrition in the other
+genera of the Entoptychinae; none developed the dental specializations
+seen in _Entoptycus_. Rootless, ever-growing cheek teeth,
+discontinuous enamel patterns, and monoprismatic molars were not
+evolved in the subfamily Geomyinae until the late Pliocene.
+
+
+Phyletic Trends in Subfamily Geomyinae
+
+The subfamily Geomyinae is made up of three groups, recognized
+taxonomically for the first time in this account as
+tribes--Dikkomyini, Thomomyini, and Geomyini (for full discussion of
+classification, see previous account). The phylogeny proposed by me is
+illustrated in Figure 3. The tribe Dikkomyini is characterized by
+generalized and primitive features that together form the basic
+structural foundation of the subfamily. Evolution within the
+Dikkomyini resulted in the acquisition and perfection of fossorial
+adaptations. The Thomomyini and Geomyini are considerably more
+specialized than the ancestral Dikkomyini from which they evolved.
+The Geomyini are clearly more specialized than the Thomomyini,
+suggesting closer affinity between the Thomomyini and the Dikkomyini
+than between the Geomyini and the Dikkomyini. The specializations in
+the dentition and the associated changes in the skull of the
+Thomomyini and Geomyini permit more efficient mastication of fibrous
+vegetation. Along with these specializations, fossorial adaptations
+inherited from the Dikkomyini are retained without noteworthy
+modification.
+
+_Dikkomys_, the earliest known genus of the tribe Dikkomyini, can be
+taken as a starting point of evolution for the subfamily Geomyinae.
+The Pliocene genus _Pliosaccomys_ is the only other known geomyine
+having primitive features closely resembling those of _Dikkomys_. The
+relatively close but previously unrecognized relationship between
+_Dikkomys_ and _Pliosaccomys_ can be understood when patterns of wear
+on the occlusal surfaces of the cheek teeth are taken into account. It
+appears that _Pliosaccomys_ descended from _Dikkomys_-like stock, if
+not _Dikkomys_ itself. Although _Dikkomys_ is towards the beginning of
+this phyletic sequence and _Pliosaccomys_ towards the end of the
+sequence, the primitive features shared by the two provide a
+generalized morphotype for the subfamily Geomyinae.
+
+In the molariform dentition, an almost complete series of stages of
+wear in _Pliosaccomys_ has been preserved, and those of _Dikkomys_ can
+be reconstructed with reasonable accuracy from those that are known
+(see Fig. 4):
+
+(1) In the initial stage of wear in _Dikkomys_ the anterior and
+posterior columns are separated by an intervening valley (Fig. 4A),
+and the occlusal surface of each column bears a loph of dentine
+surrounded by a ring of enamel: protoloph on the anterior column and
+metaloph on the posterior column of the upper teeth (protolophid and
+hypolophid in corresponding positions in the lower teeth). Actually
+this stage is not preserved in the known material of _Dikkomys_, but
+does occur in both geomyines and entoptychines in all stages of
+evolution, and it must have also occurred in _Dikkomys_ in order for
+the next two stages, which are preserved, to have developed.
+
+(2) The occlusal surfaces are ground down to a level where the enamel
+loops of the two columns join at their mid-points, thus forming an
+H-shaped pattern (Fig. 4B), or more exactly a pattern resembling a
+figure 8. Probably this was the primitive pattern in the final stage
+of wear in the geomyid ancestor of the Oligocene.
+
+[Illustration: FIG. 3. Diagram depicting geologic range and probable
+phyletic relationships of the family Geomyidae. Dashed lines represent
+parts of lineages that are not represented by fossil records, and
+solid lines represent parts of lineages verified by actual specimens.
+Question marks indicate uncertainty of suggested ancestry of known
+taxa. The relationships within the subfamily Entoptychinae are
+modified after Wood (1936), and the temporal range of the Miocene
+geomyids have been adjusted to agree with current stratigraphic
+correlations. Hence, _Pleurolicus_, _Gregorymys_ and _Dikkomys_ are
+illustrated as ranging into the Hemingfordian, rather than being
+confined to the Arikareean (see MacDonald, 1963, and Black, 1961).]
+
+(3) In the pre-final stage of wear, the anterior and posterior lophs
+of the first and second molars unite secondarily at the edge of their
+protomeres (labial side in the lower and lingual in the upper), thus
+enclosing an isolated enamel fossette (Fig. 4C). Lateral union occurs
+in the lower teeth because the vertical depth of the labial re-entrant
+angle is less than the depth of the lingual re-entrant fold. In the
+upper teeth the reverse is true. The re-entrant angle on one side of
+the premolar is as deep vertically as the angle on the other side of
+that tooth, and both reach the base of the crown; therefore, they do
+not disappear at any stage of attrition. The same pertains in the
+third lower molar.
+
+(4) In the final stage of wear (Fig. 4D), the enamel fossette
+disappears as a result of continued attrition on the occlusal surface
+in the upper series. The fossette may vary somewhat in vertical depth
+in m1 and m2, but the amount of wear required for its effacement would
+be greater than in the upper teeth. Therefore, upon wear, the
+U-pattern would become characteristic of the final stage in M1 (and
+probably also M2), but the modified H-pattern described in Fig. 4C
+would prevail in m1 and m2. Perhaps, in extremely worn teeth, the
+labial fossette of m1 and m2 would disappear. If this advanced stage
+of effacement is obtained, then the two columns would be united across
+the entire surface of their protomeres from the center of the crown to
+its labial edge, and the occlusal pattern would be in the shape of a
+U.
+
+The occlusal pattern, at least in M1 and M2, in the final stages of
+wear in _Dikkomys_ resembles that in the subfamily Entoptychinae, but
+the U-pattern develops on only the first and probably the second molar
+in _Dikkomys_ and not on all of the cheek teeth as it does in the
+entoptychines. Judging from the material that has been described, the
+U-pattern did not develop in the lower teeth of _Dikkomys_ until the
+Hemingfordian (_D. woodi_), upper Rosebud, and specimens of _D.
+matthewi_ from the earlier Arikareean, lower Harrison, suggest that
+the modified H-pattern, with secondary coalescence at the edge of the
+protomeres, persisted throughout life, without developing the
+U-pattern in the final stages of wear.
+
+Essentially the same patterns of wear characterize the genus
+_Pliosaccomys_, except that the earlier stages were telescoped and the
+second stage was omitted while another (final) stage was added. The
+stages are reconstructed in sequence in figure 4, and all are based on
+preserved dentitions, as follows:
+
+(1) The first phases of wear produced the pattern (Fig. 4E and I)
+described for _Dikkomys_ in the previous account (Fig 4A).
+
+(2) A small additional amount of wear produced the 2nd stage (Fig. 4F
+and J) characterized by a U-pattern, formed by union of the anterior
+and posterior columns at the edge of the protomeres of the first and
+second molars, both above and below, without first forming an H-shaped
+pattern. Union at the mid-points thus was omitted from the sequence of
+wear in these two teeth. In the premolars and third molars the
+primitive H-pattern did form, as in _Dikkomys_. The pattern of wear in
+the first two molars is the same as in the entoptychines of the early
+Miocene. The trend of evolution through which the _Pliosaccomys_
+lineage passed must have featured a progressively earlier union at the
+edge of the tooth until the lateral coalescence occurred
+simultaneously with the median union. At that stage, emphasis was
+shifted to the union at the edge of the tooth, and eventually the
+teeth failed to unite at their mid-points and the U-pattern developed
+directly. Therefore, the horizontally deep re-entrant fold that
+separates the two lophs of the U-pattern is equivalent to one fold
+plus the apex of the opposite fold.
+
+(3) The horizontal re-entrant fold of the U-pattern was remarkably
+shallow vertically and disappeared with little additional wear. Thus
+the two parts of M1, and also of M2, are united into a single column
+except for a slight inflection on the labial side and this is true
+also of m1 and m2 except for a slight inflection on the lingual side
+(Fig. 4G and K). The inflection appears to have persisted in the upper
+teeth (Fig. 4H), but evidently with slight wear, disappeared in the
+lower teeth (Fig. 4L). The final monocolumnar pattern was attained
+early ontogenetically, evidently before the permanent premolar had
+fully erupted; hence, the earlier stages occurred only in transition,
+persisted for only a brief interval in the teeth of juveniles, and the
+final stage developed in the young animal and lasted throughout the
+rest of its life in _Pliosaccomys_. In _Dikkomys_ the two columns
+never united into a single column, and a bilophodont occlusal pattern
+persisted throughout life.
+
+The early phyletic development of the subfamily Geomyinae took place
+in the tribe Dikkomyini from the early Miocene into the early
+Pliocene. Compared with the rapid evolution of the specializations
+that distinguish the Entoptychinae, the structural changes in the
+early Geomyinae occurred at a remarkably slow rate. In fact the
+lineage changed but little from _Dikkomys_ to _Pliosaccomys_, in parts
+of the animal that can be compared, as illustrated by the low-crowned
+and rooted cheek teeth, the continuous enamel bands, the lack of
+grooving of the upper incisor, the retention of the primitive
+H-pattern, both above and below, in the premolar and third lower
+molar, and the ridges and fossae of the mandible to which the muscles
+of mastication attach. The only major changes detected in the known
+fragments are in the pattern of wear and the final configuration of
+the first and second molars, as described above. The unification of
+the two lophs in each of these two teeth into a single column was a
+significant step in the evolution of the Geomyinae, and is a stage
+between the primitive bilophodont pattern of the early and middle
+Miocene geomyines having continuously bicolumnar teeth and the
+monolophodont pattern in the modern pocket gophers of both lineages in
+which these teeth consist of a single column in all but the initial
+stages of wear. The monocolumnar structure of the first and second
+molars in the final stages of wear, therefore, is closer to that in
+the lineage of _Thomomys_ than it is to that of _Dikkomys_. Other
+specializations in the dentition of _Pliosaccomys_, especially in m1
+and m2 where the H-pattern has been completely eliminated from the
+sequence of wear, are too far advanced for _Pliosaccomys_ to have
+given rise to the tribe Geomyini. The teeth in the immediate ancestor
+of the Geomyini must have been less specialized in m1 and m2, perhaps
+about as in _Dikkomys_. In the m1 and m2 of the tribe Geomyini, the
+H-pattern is formed in the initial stages of wear; therefore, in the
+early Pliocene ancestor, presently unknown in the fossil record, the
+H-pattern probably was present. Even so, the ancestor of the Geomyini
+and that of _Pliosaccomys_ probably were closely allied otherwise, and
+both probably had attained the highly specialized fossorial
+adaptations characterizing all modern pocket gophers, before the
+divergence of _Pliosaccomys_ and the Geomyini took place.
+
+The evidence points to a major divergence of the geomyines that lived
+in the latest Miocene or the early Pliocene (probably the latter) and
+that gave rise to the two modern lineages, Thomomyini and Geomyini
+(see Fig. 3). One, the most primitive of the two, gave rise to the
+Thomomyini lineage that eventually evolved into _Thomomys_.
+_Pliosaccomys_ is closely allied to the ancestry of this lineage,
+although it is probably not the actual ancestor, as mentioned
+previously. Aside from the aforementioned specializations of the first
+and second molars, the features of the Thomomyini are less advanced
+than in the other specialized lineage (tribe Geomyini). Primitive
+traits retained in the tribe Thomomyini (and also characteristic of
+the ancestral tribe Dikkomyini) are: (1) Small size, in general no
+larger than the ancestral morphotype; (2) lack of grooving on the
+upper incisor (although a slight rudimentary groove is developed
+rarely in some living species); (3) retention of anterior and
+posterior enamel plates in lower and upper cheek teeth; (4) premolars
+having widely open re-entrant folds; (5) smooth and generalized skull
+lacking marked angularity, regosity or cresting (neither the sagittal
+nor the lambdoidal crest are ordinarily well developed except in
+_Thomomys bulbivorus_); (6) forefoot small, less modified for digging
+than in the Geomyini.
+
+
+    [Illustration: FIG. 4. Drawings of the molariform dentitions of
+       _Dikkomys_ and _Pliosaccomys_ (Tribe Dikkomyini) depicting the
+       patterns of wear on the occlusal surfaces. Ontogenetically, the
+       stages of wear are arranged from left to right in each row. Stages
+       not represented by actual specimens have been carefully
+       reconstructed from information provided by known stages in the
+       sequence of wear and the dentitions of other geomyines. × 5.
+
+    A-D. _Dikkomys woodi_, right lower tooth-row, including p4-m3.
+         Patterns based on No. P26284 (FMNH) from Upper Rosebud (Middle
+         Miocene), Shannon Co., South Dakota (B above).
+
+    E-H. _Pliosaccomys dubius_, left upper tooth-row, including P4-M2
+         (M3 unknown). Patterns based on Nos. 1798 and 1799 (LAM) from
+         Smiths Valley (Middle Pliocene), Lyon Co., Nevada.
+
+    I-L. _Pliosaccomys dubius_, right lower tooth-row, including p4-m3.
+         Patterns based on Nos. 1796 (holotype), 1804, and 1806 (LAM)
+         from Smiths Valley (Middle Pliocene), Lyon Co., Nevada.
+    ]
+
+The lineage of the Thomomyini is essentially rectilinear and without
+the major branching seen in the tribe Geomyini. The one genus,
+_Thomomys_, appears first in the Upper Pliocene (early Blancan time),
+and the specializations characterizing the lineage had already
+developed by that time. Evidently, the early stages of divergence from
+the ancestral stock resulted in the development of rootless,
+ever-growing, more hypsodont cheek teeth, simplification of M3, and
+enlargement of the masseteric ridge on the mandible. The enamel
+investment on the sides of the molariform teeth is interrupted owing
+to intrusion of tracts of dentine on the sides of each column. Even
+so, complete anterior and posterior plates are retained on all of the
+cheek teeth (Fig. 5, K and L) and there is no trend toward additional
+loss of enamel as in the Geomyini. The enamel on the sides of the
+column has little functional value, and its elimination probably
+reduces friction during the anteroposterior movements of the lower
+jaw, thereby increasing the efficiency of the cutting blades on the
+anterior and posterior wall of the tooth. The simplification of M3 was
+achieved by union of the two columns of the primitive pattern into a
+single column and obliteration of both the labial and lingual
+re-entrant folds in the first stages of wear. The adult tooth (see
+Fig. 5L) is without trace of the bilophate pattern and is not
+elongated; therefore, its structure is essentially the same as that of
+the first and second upper molars.
+
+In the Thomomyini, the two lophs of the unworn molars unite entirely
+across the width of their surfaces with the first traces of wear (see
+Fig. 5, I and J), owing to the shallow and uniform depth of the
+transverse valley. In the molars, the final pattern is acquired,
+therefore, before the deciduous premolar has been replaced by the
+permanent tooth. A relatively shallow re-entrant inflection between
+the ends of the parameres sometimes is retained, although it also will
+disappear with slight additional wear. Therefore, both lophs tend to
+unite completely with the first stages of wear in the Thomomyini, thus
+omitting both U and H patterns from the sequence of wear. This is the
+highest degree of specialization attained in the Geomyidae in regard
+to the patterns of wear, since a sequence of bilophodont patterns
+appear in both the Dikkomyini and Geomyini before the monoprismatic
+pattern is developed.
+
+    [Illustration: FIG. 5. Drawings of molariform dentitions
+       representative of the tribes Geomyini and Thomomyini depicting
+       patterns of wear on the occlusal surface. A-D represent, in
+       ontogenetic sequence from left to right, upper tooth-rows of the
+       tribe Geomyini. E-H represent, in the same sequence of stages,
+       lower tooth-rows of the tribe Geomyini. I-L represents both upper
+       and lower tooth-rows of both pre-final and final stages of wear in
+       the tribe Thomomyini. All × 5.
+
+    A and E. _Geomys bursarius majusculus_, No. 2948 (KU), Douglas Co.,
+             Kansas. Right upper (A) including DP4-M3; lower left (E)
+             including dp4-m3.
+
+    B and F. _Pappogeomys bulleri burti_, No. 100444 (KU), 10 mi. NNW
+             Barra de Navidad, Jalisco. Right upper (B) including P4-M3;
+             right lower (F) including p4-m3 (both P4 and p4 with unworn
+             enamel caps).
+
+    C and G. _Pappogeomys bulleri albinasus_, No. 31044 (KU), 10 mi. S
+             and 8 mi. W Guadalajara, Jalisco. Right upper (C) including
+             P4-M3; right lower (G) including p4-m3.
+
+    D and H. _Pappogeomys bulleri albinasus_, No. 31002 (KU), W side La
+             Venta, 13 mi. W and 4 mi. N Guadalajara, Jalisco. Right
+             upper (D) including P4-M3; right lower (H) including p4-m3.
+
+    I and J. _Thomomys talpoides bridgeri_, No. 6865 (KU), 2 mi. up Mink
+             Creek, Pocatella, Bannock Co., Idaho. Left upper (I),
+             DP4-M3; left lower (J), dp4-m3.
+
+    K and L. _Thomomys talpoides fossor_, No. 13205 (KU), Wasson Ranch,
+             3 mi. E Creede, Mineral Co., Colorado. Right lower (K),
+             p4-m3; left upper (L), P4-M3.
+    ]
+
+Relationship of the Geomyini with the ancestral Dikkomyini is most
+clearly demonstrated in the sequence of wear on the occlusal surfaces
+of the molars. As in all geomyids, the upper part of the crown is
+biprismatic in the newly erupted tooth, and the two columns are
+separated by an intervening valley. With slight attrition on the
+unworn enamel cap, the weakly developed cusps merge and form a
+transverse enamel loop on each of the two columns (see third molar in
+Fig. 5, A and E), each loop enclosing a core of dentine that had
+become exposed. The valley between the two columns is shallow, and
+upon further wear of the tooth, the two loops unite. The two columns
+become joined at different points in the upper and lower molars
+depending on the varying depth of the valley in different teeth.
+Therefore, upper and lower molars develop distinctly different
+occlusal configurations.
+
+In the lower molars, the pattern characteristic of _Dikkomys_ (Fig.
+4C) is preserved without significant modification, as illustrated in
+an immature specimen of _Geomys_ (see Fig. 5E). The H-pattern and
+modified H-pattern are developed in the same sequence of wear in the
+Geomyini. A juvenal female (not illustrated), KU 2931, provides an
+example of the intermediate H-pattern. In this specimen, the
+protolophid and hypolophid of the left m2 are united only at their
+mid-points, indicating that the pattern of wear occurs in the same
+sequence in the Geomyini as it did in the Miocene genus _Dikkomys_.
+After the two columns have become united at their mid-points, a
+secondary union is formed at the edge of their protomeres, thus
+enclosing the enamel fossette as illustrated in Figure 5E (this is the
+modified H-pattern mentioned above). However, the fossette itself is
+shallow and soon disappears with slight wear. At this stage, the
+occlusal configuration would be in a U-pattern (m1 in Fig. 5E). The
+lingual re-entrant fold is also shallow in vertical depth; therefore,
+it is obliterated by wear following the eradication of the labial
+fossette. Consequently, the two columns are united into one. In m3
+(see Figs. 5E, F, and G), the two columns merge by progressive lateral
+expansion of the medial isthmus.
+
+In the first and second upper molars, the two columns unite across the
+entire surface of their protomeres from near the lingual edge of the
+crown to near its center. A minute inner inflection may be temporarily
+retained in some teeth. At this stage (see Fig. 5B), the parameres are
+still separated by the labial fissure, and the occlusal pattern is in
+the shape of a U, resembling, but not exactly duplicating, the
+pre-final pattern of Ml and M2 in the genus _Pliosaccomys_ (see Fig.
+4H). The labial fissure is shallow, and, with further wear, the
+inflection is worn away and the parameres also unite, thereby forming
+a monoprimatic crown in the final stage. In M3, the two lophs first
+become united near the edge of their protomeres (see Fig. 5B),
+therefore forming a U-pattern similar to that developed in Ml and M2
+of _Pliosaccomys_. The connection of the two lophs is not directly at
+the end of the protomere; consequently a shallow lingual inflection
+remains. The lingual edge of the valley is also shallow, and, with
+continued wear a second union of the two lophs takes place near the
+ends of their parameres, and the deeper, interior part of the valley
+remains as an isolated enamel fossette (see Fig. 5C). The two primary
+lophs of the tooth are now joined near both sides, having shallow
+lingual and labial re-entrant angles on the sides and the enamel
+island in the center. With continued effacement of the occlusal
+surface, the fossette will be eradicated, and the pattern of the
+occlusal surface will become the partially biprismatic pattern of the
+final stages (adult) of wear (see Fig. 5D). M3's of _Dikkomys_ and
+_Pliosaccomys_ are not known; however, it seems reasonable to assume
+that the pattern of wear in the M3 of Dikkomyini was not essentially
+different from that of the Geomyini, except that it is likely that the
+U-pattern of the second stage of wear in the Geomyini was probably the
+final stage in the genus _Dikkomys_.
+
+Judging from the pre-final stages of wear, the dentition of the
+Geomyini provides a curious combination of patterns that resemble in
+part the Miocene genus _Dikkomys_ and in part the early and middle
+Pliocene genus _Pliosaccomys_. There is no significant variation in
+the premolars or third molars (at least in the lower teeth) of the
+Geomyinae from the early Miocene to late Pliocene; therefore,
+deviations of major significance are in the character of the first and
+second molars. In the Geomyini, the patterns of wear of m1 and m2 are
+the same as those of _Dikkomys_, and are distinctly different from
+those of _Pliosaccomys_ where the two columns first unite at the edge
+of their protomeres to form a U-pattern, rather than at their
+mid-points to form an H-pattern. Even though the intermediate stages
+of ontogeny in m1 and m2 of _Pliosaccomys_ and the Geomyini are
+entirely different, the bicolumnar crowns of both eventually unite,
+upon wear, into a single column. On the other hand, the patterns of M1
+and M2 in the Geomyini most closely resemble those of _Pliosaccomys_,
+rather than _Dikkomys_. In this regard it should be pointed out that
+the upper molars of _Dikkomys_ are presently represented by only one
+tooth, an M1 in an early stage of wear. As described already, the
+patterns of M1-2 evidently would be mirror images of m1-2 in
+corresponding stages of wear. However, the initial union of the two
+columns, in the M1 that is known, is somewhat to the lingual side of
+center and the relatively small lingual valley does not reach the base
+of the crown, indicating, that eventually with wear, the two columns
+of _Dikkomys_ might have become united across the entire surface of
+their protomeres as in _Pliosaccomys_. Even so, the two columns of M1
+do initially join closer to their mid-points than they do in
+_Pliosaccomys_, and, if they did actually unite across their
+protomeres, the union would have occurred with subsequent wear. That
+is, the first occlusal pattern would be H-shaped (but with the
+connection closer to the lingual than the labial side), as in m1 and
+m2, and it would become U-shaped only after additional wear. This
+sequence of patterns of M1 and M2, as already pointed out, does not
+pertain in _Pliosaccomys_ or the Geomyini, since the U-pattern is
+formed with the first union of the two columns at the edge of their
+protomeres, and the primitive H-pattern is never developed, unless one
+counts the slight lingual inflection, that occasionally is formed just
+after the two columns unite, as being indicative of the primitive
+pattern. As in the lower teeth, the bicolumnar crowns of early
+ontogeny in both _Pliosaccomys_ and the Geomyini become eventually
+united, with wear, into a single column.
+
+Based upon the foregoing evidence, it would seem likely that the
+Geomyini evolved from an early Pliocene (perhaps late Miocene)
+Dikkomyini ancestor that had evolved the specializations of M1 and M2
+that characterize its relative, _Pliosaccomys_, but had not also
+evolved the specializations of m1 and m2 that distinguish
+_Pliosaccomys_. Therefore, the ancestor of the Geomyini differed from
+the _Pliosaccomys_-Thomomyini lineage in its retention, unmodified, of
+the primitive patterns in m1 and m2 that characterized the earliest
+known Geomyines (_Dikkomys_). The same patterns are preserved in m1
+and m2 of its modern descendents, the living Geomyini. In the
+_Pliosaccomys_-Thomomyini lineage the pattern of m1 and m2 are
+entirely different, as described above.
+
+The earliest record of the Geomyini is the extinct genus _Pliogeomys_
+(see Fig. 6) in the latest Hemphillian (middle Pliocene) and earliest
+Blancan (late Pliocene). _Pliogeomys_ is more primitive than any
+modern genus of the Geomyini, seems to have been a late survivor of
+the primitive stock, but was itself probably a collateral lineage and
+not on the direct line of descent. The cheek teeth in _Pliogeomys_ are
+rooted and less hypsodont than in the late Pliocene examples of the
+modern genera, and the anterior enamel plate of the lower molars shows
+no indication of reduction, as would be expected if _Pliogeomys_ were
+in the direct line of evolution. Separation of _Pliogeomys_ from the
+main stem of the Geomyini probably occurred after several
+specializations had already been achieved by the Geomyini. Two
+inheritances might have been grooving on the upper incisors and some
+reduction in amount of enamel on the sides of the cheek teeth. The
+dentine tracts on the sides of the cheek teeth of _Pliogeomys_ are
+narrow (see Fig. 7A) and barely separate the enamel blades and there
+is no discernible reduction in the anterior enamel blades on its lower
+molars. Those blades evidently were lost in the main lineage before
+the Pleistocene radiation of the living genera took place.
+_Pliogeomys_ is in an intermediate stage in evolution, and was not so
+advanced as was the main lineage at the time _Pliogeomys_ died out.
+Its structure does provide clues as to phyletic development that took
+place in the main lineage.
+
+Specialized trends in the early phylogeny of the Geomyini included:
+development of rootless, ever-growing cheek teeth and an increase in
+hypsodonty; loss of the bicolumnar structure of the first and second
+molars, and, consequently, the formation of a single elliptical column
+in the final stage of wear; interruption of the enamel investment of
+the molariform teeth and formation of anterior and posterior enamel
+plates; and enlargement of the masseteric ridge and fossa. Each of
+these trends occurred independently in the Thomomyini, and each is an
+example of parallelism in the phyletic evolution of the two lineages.
+Three additional specializations lacking in the Thomomyini are the
+grooving on upper incisors, loss of anterior enamel plate in lower
+molars, and development of a basitemporal fossa on the mandible.
+Evidently, two grooves evolved in the ancestral incisors in the same
+bisculcate pattern preserved in _Pliogeomys_, _Zygogeomys_ and
+_Geomys_. The innermost groove is weakly developed in _Pliogeomys_,
+suggesting that this character was in an intermediate stage of
+evolution in the ancestral lineage at the time that _Pliogeomys_ split
+off. Numerous other specializations in the Geomyini appeared later,
+but evolved in the different genera that diverged from the ancestral
+lineage and are discussed separately in the next account. Only two of
+the major features characterizing the Dikkomyini are retained in the
+Geomyini: the H-pattern on the occlusal surface of the m1 and m2
+developed during the initial stages of wear, and the bicolumnar
+pattern of M3. Adaptive radiation produced the living genera of the
+Geomyini in the late Pliocene and early Pleistocene (see Fig. 6) and
+subsequent specialization of the ancestral morphology followed.
+
+Parallelism in the molars of later geomyines and the Entoptychinae is
+illustrated by the lateral interruption of the enamel investment and
+loss of enamel plates and by the omission of the H-pattern stage in
+the first and second molars (in _Pliosaccomys_). Resemblance of
+dentitions in certain stages of wear in _Pliosaccomys_ and in
+entoptychines led some investigators, for instance, Hibbard
+(1953:357), to suggest that _Pliosaccomys_ descended from one of the
+less specialized entoptychines, possibly _Grangerimus_ but probably
+_Gregorymys_. Actually, the highly specialized upper and lower
+premolars and third molars of the entoptychines rule them out as
+ancestors of the later geomyines. The evolution of entoptychine-like
+features in _Pliosaccomys_ is regarded as an example of iteration, a
+pattern of parallelism (see Simpson, 1953:248-253) where an
+allochronic and independent lineage undergoes the same evolutionary
+trend that phyletically characterized an earlier lineage, usually
+after the latter has become extinct. In this case, the lineage giving
+rise to _Pliosaccomys_ passed through the same phyletic stages in its
+evolution in the early Pliocene (and possibly the late Miocene) as did
+the entoptychines in the late Oligocene and early Miocene.
+
+Another parallelism by iteration, occurring in the middle and late
+Pliocene in both the Thomomyini and Geomyini, is the loss of enamel
+from the lateral surfaces of the cheek teeth, and, in the Geomyini
+only, the eventual loss of the anterior plate in the lower teeth and
+the posterior plate in the upper teeth. Both features were evolved
+more than an epoch earlier in the specialized entoptychid genus
+_Entoptychus_ of the lower Miocene. In _Entoptychus_, only the
+posterior plate of the lower molars and the anterior plate of the
+upper molars remained in the final stages of attrition, although a
+central enamel fossette, a remnant of the re-entrant fold, remained
+throughout life. Iteration is also expressed in the subfamily
+Geomyinae by the development of grooving on the upper incisor and the
+formation of the basitemporal fossa. A shallow but distinct
+basitemporal fossa occurs between the coronoid process and the third
+lower molar in the genus _Entoptychus_ and a sulcated upper incisor, a
+single shallow groove usually near the median border of the tooth, is
+found in the genus _Gregorymys_ of the subfamily Entoptychinae. Both
+features are regarded as advanced specializations in the tribe
+Geomyini, even though each was evolved in the entoptychines of the
+Lower Miocene.
+
+The postcranial skeleton of living genera of pocket gophers, as befits
+animals that spend most of their life within underground burrows, are
+highly specialized for a fossorial life. Elements of the postcranial
+skeleton recovered from Lower Miocene deposits indicate that the
+entoptychines were only semi-fossorial (see Cope, 1884:857; Wood,
+1936:4-5; Wilson, 1949:117-118). One of the basic trends of the
+entoptychines was towards greater fossorial adaptation; the skeleton
+of _Entoptychus_ shows a greater degree of fossorial adaptation than
+earlier genera of the subfamily. There is no reason to suppose that
+the geomyine genus _Dikkomys_, which lived at the same times as the
+entoptychines, had acquired any more advanced fossorial adaptations
+than had the entoptychines.
+
+The most pronounced fossorial adaptations seem to have evolved only in
+the ancestral lineage of the modern geomyines, probably in the latter
+part of the Miocene and in the early Pliocene, before the modern
+Thomomyini and Geomyini diverged. Extreme fossorial adaptations in
+herbivorous rodents, such as those characteristic of the modern pocket
+gophers and their immediate ancestors, are thought to have evolved
+only in response to pronounced arid conditions. The Entoptychinae and
+evidently the early geomyines lived in environments that were either
+tropical or temperate, and under conditions more mesic than I would
+consider necessary to bring about selection pressure resulting in
+fossorial specializations. In late Oligocene and early Miocene,
+according to Axelroad (1958:433-509), arid conditions did not exist in
+the United States, and the only xerophytic environments in North
+America occurred on the Central Plateau of México. Moreover (Axelroad,
+_loc. cit._), arid conditions did not develop in the western United
+States until the early Pliocene. Geomyids evidently became extinct in
+this region at the close of the Middle Miocene, and none appear in
+fossil deposits in the western United States until the latest Lower
+Pliocene (Clarendonian). The reappearance of geomyids, _Pliosaccomys_,
+in the western United States coincides with a trend toward aridity and
+the northward movement of the Madro-tertiary geoflora into the Great
+Basin and Great Plains from its place of origin on the Central Plateau
+of México (Axelroad, _loc. cit._). Later, in the middle and later
+Pliocene, the Madro-tertiary geoflora gave rise to the modern
+xerophytic plants that now characterize the desert vegetation of North
+America.
+
+The Madro-tertiary climax does not appear as a major flora until the
+Miocene, but probably originated earlier. According to Axelroad (_loc.
+cit._), this xerophytic flora evolved from elements of the
+Neotropical-tertiary geoflora that became adapted to arid conditions
+that developed in the rain shadow of the high mountains flanking the
+Central Plateau of México. Originally, the Madro-tertiary flora
+consisted of small trees, shrubs, and grasses. Although some elements
+of this flora moved northward in the late Miocene, the major part of
+it remained in México until the early Pliocene. In the western United
+States, mountain formation increased in intensity in the Pliocene and
+continued on into the early Pleistocene. As the mountains became more
+elevated, especially the Sierra Nevada and Cascade ranges, they
+blocked the prevailing winds from the Pacific Ocean and extensive
+aridity developed on their leeward side. As xeric conditions became
+widespread, the Madro-tertiary flora successfully occupied the drier
+regions of southern California, the Great Basin, and the western parts
+of the Great Plains.
+
+While the Entoptychinae probably evolved in response to the
+Arcto-tertiary flora, the late Tertiary geomyines probably evolved in
+response to the Madro-tertiary geoflora on the Central Plateau of
+México. Some of these early geomyines, especially ancestors of the
+modern lineages, probably were pushed southward by competition with
+the more specialized entoptychines. Most geomyines were pushed out of
+the northern area of distribution, except for _Dikkomys_ that survived
+in association with the entoptychids throughout the early and middle
+Miocene. During this time, and probably continuing on into the late
+Miocene, the geomyines occurring to the south in México became adapted
+to the arid environments of the Madro-tertiary geoflora.
+
+Of course, information is lacking about climates in several parts of
+the late Miocene and early Pliocene. When such information becomes
+available it conceivably could modify the hypothesis outlined
+immediately above.
+
+The principal trend of evolution in these semi-fossorial rodents was
+toward more complete fossorial adaptation, and the pronounced
+fossorial features characteristic of the modern pocket gophers were
+perfected. This trend continued in response to the intense selection
+pressures in this arid environment. The principal structural
+characters effected were in the postcranial anatomy, especially in the
+skeletal and muscular systems. Consequently, it is not surprising that
+in skull and dentition, _Pliosaccomys_ differs but little from
+_Dikkomys_. Therefore, most of the basic structural specializations so
+far developed for subterranean existence probably had evolved by the
+time geomyines moved back north in the early Pliocene. Both modern
+lineages, the tribes Thomomyini and Geomyini, have essentially the
+same fossorial features, and it seems unlikely that these features
+were acquired independently in the relatively short period of time
+available to them after their divergence; probably they were inherited
+from a common ancestor. These probabilities indicate that the
+evolution of the fossorial specialization was in the later phyletic
+development of the tribe Dikkomyini.
+
+
+Plio-Pleistocene radiation of Geomyini
+
+Unlike the lineage of the Thomomyini that remained essentially
+rectilinear through out its history, the Geomyini in the late Pliocene
+and the early Pleistocene underwent adaptive radiation in a degree
+comparable to the earlier radiation of the Entoptychinae, and all of
+the later history of the tribe is dominated by the radiation--the
+resulting structural diversity. At least four lineages were produced
+by the Plio-Pleistocene radiation (see Fig. 6); each originated at
+essentially the same time (late Pliocene) presumably from the same
+ancestral stock. Each of these lineages within the Geomyini has given
+rise to one of the four modern genera: _Zygogeomys_, _Geomys_,
+_Orthogeomys_, and _Pappogeomys_.
+
+    [Illustration: FIG. 6. Plio-Pleistocene radiation of the Tribe
+       Geomyini.]
+
+
+_Morphotype_
+
+The immediate, unknown, ancestor probably lived on the Central Plateau
+of México. After the radiation began the ancestors of _Geomys_ and
+_Zygogeomys_ extended their ranges northward.
+
+Features of the hypothetical morphotype, that would permit derivation
+of the modern genera would include the following: (1) Skull
+generalized, neither excessively long and narrow or short and broad;
+(2) skull smoothly rounded, without pronounced angularity, rugosity or
+cresting (sagittal crest probably lacking, even in old individuals);
+(3) zygomata slender, without lateral platelike expansions; (4)
+rostrum moderately broad; (5) upper incisors bisulcate, two grooves in
+pattern found in _Pliogeomys_, _Zygogeomys_ and _Geomys_; (6) lateral
+re-entrant angles of premolars obtuse; (7) p4 having four enamel
+plates (one on anterior wall, one on posterior wall, and two lateral
+plates) and lower molars having one enamel plate on the posterior wall
+of tooth (anterior plate is lacking); (8) P4 having four enamel
+plates, in same pattern as described for p4, M1 having two enamel
+plates (one anterior and one posterior), M2 same as M1, M3 having
+three plates (one anterior, two lateral on sides of posterior loph,
+none posterior); (9) M3 subtriangular in cross-section, distinctly
+bicolumnar, two columns marked by shallow re-entrant folds and
+connected by broad isthmus; (10) masseteric ridge large, forming high
+crest bordering masseteric fossa; (11) basitemporal fossa shallow;
+(12) angular process of mandible short, its lateral projection barely
+exceeding that of zygomatic arch.
+
+
+_Specializations in Genera_
+
+In relation to the primitive morphotype, increase in size,
+simplification of dentition, and changes in shape of skull are
+regarded as specializations. Considerable parallelism between the four
+lineages is seen. But each lineage is distinguished by a combination
+of specialized features, and three by a few unique specializations.
+
+Among trends resulting in simplification of the dentition, reduction
+of enamel on the posterior wall of the upper cheek teeth has occurred
+in various degrees in all lineages of the Geomyini even to loss of all
+enamel on the posterior wall of the premolars and molars in two
+genera. Loss of some enamel is more common on P4 than on M1-2, and has
+occurred in all genera (see Figs. 7 and 9.)
+
+In evolutionary sequence loss of enamel from M1 and M2 usually occurs
+after, but never preceding, the reduction of enamel on P4. Loss of
+enamel plates from the posterior face of M1 and M2 is associated with
+the evolution of an efficient anterotransverse shearing action of the
+teeth.
+
+On the anterior wall of those teeth no reduction of the cutting blade
+has been observed; a complete anterior plate is retained in all living
+Geomyini.
+
+Presence of both the posterior and anterior plates decreases the
+efficiency of transverse shearing, by providing two upper plates
+(anterior plate of one tooth and posterior plate of the preceding
+tooth) over which the lower cutting blade _simultaneously_ must pass
+with each movement. The advantages of shearing over the more common
+mechanics of planing are largely lost unless the posterior plates are
+eliminated. Also, none of the living Geomyini have retained a
+definitive posterior enamel plate on M3, the last upper molar; but two
+well-developed lateral plates, that extend almost all of the way back
+to the posterior apex of M3, have been retained, and, together
+function as a posterior plate. Loss of either or both of the lateral
+plates of M3 is rare, and occurs only in old individuals. Their loss
+in the final stages of wear may represent the beginning of a new trend
+in those species where it occurs (the _castanops_-group of the
+subgenus _Cratogeomys_). In any case, reduction of enamel takes place
+by transverse shortening of the plate through the complete loss of
+enamel on one end, the diminution beginning first on the labial end
+and proceeding by progressive atrophy to the lingual end of the plate.
+Evidently, when enamel has been eliminated from the labial end of a
+plate, the rate of loss decreases markedly, and the last stages of
+evolution, terminating in complete loss of an enamel plate, occurs
+more slowly. Evolution may be arrested before complete loss has
+occurred, and that part of the enamel that remains forms a short,
+vestigial plate restricted to the lingual one-fourth or one-third of
+the wall. The enamel pattern of the lower dentition is the same in all
+of the diverging lineages, with no evidence of additional loss of
+enamel from that which had already occurred in their common ancestor
+(see Figs. 7 and 9). Reduction and loss of enamel plates began and was
+terminated in the lower dentition before reduction began in the upper
+dentition.
+
+Other dental specializations have occurred in the shape of the third
+upper molar and in the pattern of grooving in the upper incisor.
+Unlike M3 of the Thomomyini, that of the Geomyini differs in shape
+from M2, and its enamel investment differs from that of M2.
+Primitively, M3 was probably subtriangular in cross-section, and the
+posterior loph evidently projected posteriorly as a short, rudimentary
+heel that formed the apex of the triangle. Other shapes of M3 are
+considered to be specializations that have been derived from the
+primitive form. In addition to the primitive subtriangular pattern,
+the M3 of living Geomyini may be suborbicular, quadriform, elongate,
+or obcordate in shape. Usually each lineage is characterized by only
+one pattern, but in one genus (_Pappogeomys_) all patterns occur. Of
+the different forms, the elongate and obcordate seem to be the most
+highly specialized deviations from the triangular-shaped tooth. The
+bicolumnar pattern is accentuated in the elongate type (Fig. 7D, F, H)
+by deep lateral re-entrant folds, on both the lingual and labial
+sides, and by the elongation of the posterior loph into a pronounced
+heel. Teeth having this pattern have been illustrated by Merriam
+(1895:76-82) in Figures 27 (6 and 7), 28 (c and d), 34 (7 through 15),
+and 35 (8).
+
+    [Illustration: FIG. 7. Molariform dentitions of the Tribe Geomyini.
+       Drawings illustrating enamel patterns characteristic of
+       _Pliogeomys_, _Zygogeomys_, and the subgenera of _Orthogeomys_
+       (_Orthogeomys_, _Heterogeomys_ and _Macrogeomys_). × 5.
+
+          A. _Pliogeomys buisi_, No. 29157 (UMMP), holotype, Buis Ranch
+             (Upper Middle Pliocene), Beaver Co., Oklahoma. Right lower,
+             p4-m2 (m3 unknown).
+
+    B and C. _Zygogeomys trichopus trichopus_, adult female, No. 51971
+             (FMNH), Mt. Tancítaro, 10,500 ft., Michoacán. Left upper
+             (B), P4-M3; right lower (C), p4-m3.
+
+    D and E. Subgenus _Orthogeomys_. _Orthogeomys grandis guerrerensis_,
+             adult female, No. 39807 (KU), 1/2 mi. E La Mira, 300 ft.,
+             Michoacán. Left upper (D), P4-M3; right lower (E), p4-m3.
+
+    F and G. Subgenus _Heterogeomys_. _Orthogeomys hispidus hispidus_,
+             adult female, No. 23975 (KU), 4 km. W Tlapacoyan, 700 ft.,
+             Veracruz. Left upper (F), P4-M3; right lower (G), p4-m3.
+
+    H and I. Subgenus _Macrogeomys_. _Orthogeomys heterodus
+             cartagoensis_, adult female, No. 60664 (KU), Rancho
+             Redando, Volcán Lrozá, Prov. San José, Costa Rica. Left
+             upper (H), P4-M3; right lower (I), p4-m3.
+    ]
+
+
+
+The subcordate form is characterized by pronounced anteroposterior
+compression, and retention of a distinct labial re-entrant fold. The
+posterior loph apparently has been rotated in such a way that what was
+previously its posterior border now lies on the outer margin of the
+tooth; therefore, the axis of the posterior loph is strongly oblique
+in relation to the anteroposterior bearing of the maxillary tooth-row,
+and the median enamel plate also has been rotated and so lies
+transversely across the posterior wall of the tooth. Owing to the
+rotation of the posterior loph, the apex of the obcordate tooth is at
+its lingual side. The subcordate type is illustrated by Merriam (_loc.
+cit._) in Figures 27 (3 and 4), 28 (a and b), 34 (3 and 4), and 35 (5,
+6, and 7). The suborbicular and quadriform types are less specialized
+than the two described above. Both are characterized by reduction,
+often obliteration, of the bicolumnar pattern of the subtriangular
+ancestral form, especially marked by the decrease in depth of the
+lateral re-entrant folds and the decrease in length of the posterior
+projection of the posterior loph. With these changes, the tooth
+becomes essentially monocolumnar, its occlusal surface oval in outline
+in one and squarish in shape in the other. Occlusal views of the
+suborbicular form are presented by Merriam (_loc. cit._) in Figure 33
+(1, 5, 6, 7, 11, and 12) and the quadriform tooth is depicted in
+Figure 29. Grooved upper incisors are characteristic of the living
+Geomyini, but variation occurs in the number of grooves, and, if only
+one groove is present, its position on the anterior face of the tooth
+varies. Except for the previously mentioned (p. 480) abnormal tooth
+having three grooves, incisors with no more than two grooves are found
+in these pocket gophers, and this number of grooves is taken to be
+primitive. Loss of one or the other of the two grooves of the
+bisulcate pattern, therefore, is regarded as specialization. However,
+complete loss of both grooves never occurs in the Geomyini. Each of
+the four major lineages is characterized by one of the three patterns
+of grooving, and the particular groove-pattern is remarkably stable in
+each group.
+
+Shape of skull varies from dolichocephalic to platycephalic. The
+morphology of each has been described in foregoing accounts. The
+dolichocephalic skull is highly specialized for planing, a grinding
+action of the teeth; whereas, the platycephalic skull is highly
+specialized for shearing, a slicing action of the teeth. Of course,
+concomitant specializations of the dentition, as described above, are
+closely associated with both specialized trends in the skull. Most
+kinds of living Geomyini have generalized skulls that show no tendency
+toward either of the specialized conditions.
+
+Increase in size of body and skull is seen in most Pleistocene
+lineages of the Geomyini. Judging from the smallness of the skull in
+late Pliocene species, representing the base of three of these
+lineages, the ancestral species of the living assemblage were no
+larger than the living species of the subgenus _Pappogeomys_ or the
+smaller subspecies of _Geomys bursarius_. The recorded range of
+variation in condylobasal length is 36.1 to 45.5 in _Pappogeomys
+bulleri_, including both adult males and females. Probably the skulls
+of the ancestral species were not significantly larger. Maximum
+dimensions of males in living species are 74.5 (subgenus
+_Cratogeomys_) and 75.0 (subgenus _Orthogeomys_). These are more than
+twice the minima observed in _Pappogeomys bulleri_.
+
+
+Zygogeomys
+
+This is the least specialized and most primitive of the four lineages,
+has a generalized type of skull, two grooves on the anterior face of
+each upper incisor, an enamel plate on the posterior wall of P4, open
+or divergent lateral re-entrant angles on the premolars, and a
+bicolumnar and elongated M3. All of these features are primitive and
+essentially as in the ancestral morphotype. No other modern genus
+retains so much of the primitive structure. Phyletic trends in
+_Zygogeomys_ are not well documented in the fossil record; and only a
+few fossils are known and they are fragmentary as discussed before.
+The genus is represented in the late Pliocene (_Z. minor_), middle
+Pleistocene (_Z. persimilis_), and Recent (_Z. trichopus_). The living
+species is a relict population in the mountains of Central México.
+Judging from the known material, the phyletic trends in the genus have
+been increase in size, reduction of enamel on the posterior face of P4
+(occurring only in the living species) where a short enamel plate is
+retained on the lingual side of the tooth (see Fig. 7B), loss of the
+outer fourth of the enamel blade on the posterior wall of M1 and M2
+(also occurring only in the living species), development of a more
+pronounced heel on the M3 by progressive elongation of the posterior
+loph, reduction in size of the jugal and its displacement ventrally,
+which allows the maxillary and squamosal bones to meet along the
+dorsal border of the zygomatic arch. The last specialization is seen
+in at least one taxon of _Orthogeomys_ (_Orthogeomys cherriei
+costaricensis_). In my opinion, too much weight has been given to this
+feature in past classifications. Reduction of enamel in the upper
+dentition evidently occurred in the late Pleistocene, since the
+posterior plates on the upper cheek teeth were complete in specimens
+from the middle Pleistocene (_Z. persimilis_).
+
+
+Geomys
+
+_Geomys_, slightly more specialized than _Zygogeomys_, must also be
+regarded as one of the most primitive of the living genera. Primitive
+features that have been retained are the generalized type of skull,
+the bisulcate pattern of grooves on the upper incisor, and the
+retention of enamel plates on both the anterior and posterior walls
+of M1 and M2 (see Fig. 9A). All of these primitive features are
+shared with _Zygogeomys_. In addition, three other trends, or
+specializations, in evolution characterize the phyletic development of
+_Geomys_. One major trend is toward loss of the enamel plate from the
+posterior wall of P4. No trace of enamel remains on the posterior wall
+of this tooth in late Pleistocene or Recent species of _Geomys_, and
+at least one of the earlier species (_quinni_) was also characterized
+by loss of this enamel plate. Secondly, M3 retains only a vestige of
+the primitive bicolumnar pattern after the initial stages of wear. In
+most Recent specimens, especially of the species _G. bursarius_, the
+lateral re-entrant fold and the heel of M3 are small, and the
+re-entrant inflection is hardly evident. The lateral fold is more
+frequently well-developed in Irvingtonian species than in living
+species (White and Downs, 1961:13), illustrating progressive loss of
+the bicolumnar pattern in Pleistocene evolution. A third trend
+involves the modification of the lateral folds of the premolars.
+Primitively the angles of these folds are broadly open or divergently
+V-shaped, and some of the earliest species of _Geomys_, for example
+_G. quinni_, have retained this feature throughout life. Nevertheless,
+the main trend is toward progressive compression of the folds
+resulting in their walls being more nearly perpendicular, and
+parallel, to the long axis of the tooth. Obtuse re-entrant angles
+persist in premolars of young individuals of Irvingtonian species, but
+the adults are characterized by well-compressed folds, as in Recent
+species.
+
+Remains of _Geomys_ are abundant, especially from Pleistocene deposits
+of the Great Plains, but in most instances specific assignment is
+difficult or impossible since only isolated teeth or fragments of
+skulls have been preserved. Estimates of phyletic relationships of the
+known species of _Geomys_ are depicted in Figure 8; those estimates
+are useful in discussing the phyletic development of the genus. One of
+the earliest known species, _Geomys quinni_, ranges from Upper
+Pliocene to the later stages of the Lower Pleistocene (Aftonian
+interglacial deposits). The dentition of _G. quinni_ is essentially
+the same as in the living species except that open lateral re-entrant
+angles are retained in the premolars. _Geomys paenebursarius_, also of
+the early Pleistocene, is a smaller species and seems to be more
+directly in the line of evolution of the modern species. As yet
+unnamed smaller species of _Geomys_ from the Rexroad fauna (late
+Pliocene) and Saunders fauna (latest Aftonian) may also be on the main
+line of evolution. Surprisingly, _Geomys tobinensis_ and _Geomys
+garbanii_ of later Irvingtonian provincial age are less specialized
+than either _Geomys quinni_ or _Geomys paenebursarius_. It is likely
+that _G. tobinensis_ and the unnamed species from the Dixon are closer
+to the main line of descent than _G. paenebursarius_ suggesting that
+the direct ancestral lineage of the living species of _Geomys_ was
+more conservative and less specialized than _Geomys paenebursarius_ of
+the Lower Pleistocene. _Geomys quinni_ and _G. paenebursarius_ seem to
+have acquired specialized dental features in the early Pleistocene.
+_Geomys quinni_ was successful on the Great Plains, and persisted into
+the late Blancan. The main line may be represented in the early
+Pleistocene by _Geomys paenebursarius_ from the Hancock formation of
+the Texas Trans-Pecos. The structure of _G. paenebursarius_ indicates
+that it is in or close to the main line of descent, and probably
+evolved from one of the more primitive late Pliocene species of
+_Geomys_ from the Rexroad fauna.
+
+    [Illustration: FIG. 8. Tentative arrangement of species of the genus
+       _Geomys_, depicting phylogenetic trends and probable relationships
+       within the genus.]
+
+Isolated teeth, to which the name _Geomys bisulcatus_ probably
+applies, from Illinoian deposits on the Great Plains, show that the
+dentition characteristic of the living _Geomys_ had been developed by
+that time. Actually, the Illinoian material is too fragmentary to
+show clearly its taxonomic or phyletic affinities with the species of
+the later Pleistocene. Even so, the two main stocks of living
+_Geomys_, _G. bursarius_ and _G. pinetis_, had certainly been
+differentiated by Sangamon time. The other living species evidently
+evolved from one or the other of these two stocks in a period of
+isolation from the main population, probably in either the Wisconsin
+or post-Wisconsin. For example, _Geomys arenarius_ clearly
+differentiated from populations of _Geomys bursarius_ that were
+isolated by the eastward retreat of the main population from the
+southwestern United States as that region became more arid in the
+post-Wisconsin.
+
+In review, it seems that the Recent species, represented basically by
+_bursarius_ and _pinetis_, evolved from Illinoian species (_Geomys
+bisulcatus?_), which descended in turn from the more primitive species
+of the early Pleistocene, possibly _Geomys paenebursarius_ or possibly
+from descendants of the Saunders species. Actually the Saunders
+species may prove to be _Geomys paenebursarius_. At any rate, three
+trends that took place during the Pleistocene stage of evolution, in
+the direction of the modern species, were an increase in size,
+progressive loss of the posterior enamel plate on P4, and a decrease
+in the vertical depth of the enamel cap as a result of which the
+dentine is reached in the initial phases of attrition on the tooth of
+a juvenile. _Geomys garbanii_, occurring at the periphery of the range
+of the genus, is regarded as a sterile offshoot of the primitive
+_tobinensis_-line of evolution.
+
+
+Orthogeomys
+
+This is one of the more specialized genera of the Geomyini. Save for
+one record in the late Pleistocene (_Orthogeomys onerosus_), there is
+no fossil history of the genus upon which to reconstruct its
+phylogeny; therefore, its phyletic development must be estimated by
+comparing it and the primitive morphotype of the tribe. Results of
+that comparison suggest that _Orthogeomys_ has closer affinities with
+_Zygogeomys_ than with any of the other genera, and that _Orthogeomys_
+may have originated in an early dichotomy of primitive _Zygogeomys_
+stock instead of descending from the ancestral stock of the tribe.
+Except for the unisulcate incisors and the longer posterior loph on
+the third upper molars, the teeth of the two genera do not differ
+significantly. As in _Zygogeomys_, the enamel blade on the posterior
+wall of P4 has been reduced to a short plate restricted to the lingual
+third of the tooth (see Fig. 7F and H). In _Orthogeomys_, the trend in
+reduction of enamel is carried to its extreme only in the subgenus
+_Orthogeomys_, where this plate has been completely lost in most taxa
+(see Fig. 7D). The most significant trends in _Orthogeomys_, and the
+principal basis for recognizing the genus, are the dolichocephalic
+specializations of the skull, as described elsewhere, and the adaptive
+traits that have equipped the genus for living in tropical
+environments. The dolichocephalic features are more sharply defined in
+the subgenera _Orthogeomys_ and _Macrogeomys_, and are less developed
+in the subgenus _Heterogeomys_. Aside from the general dolichocephalic
+specializations, trends in _Orthogeomys_ include: Increase in size;
+loss of the median one of the two grooves on the anterior face of the
+upper incisor in the ancestral stock; increase in the anteroposterior
+length of each of the cheek teeth, as well as the aforementioned
+elongation of the posterior loph of M3; compression of the lateral
+angles of the premolars; and the remarkable increase in the size of
+the rostrum.
+
+
+Pappogeomys
+
+The genus _Pappogeomys_, as it is conceived of in this study, is
+comprised of two subgenera; one, _Pappogeomys_, is generalized and
+primitive, and the other, _Cratogeomys_, is specialized, and includes
+the most highly specialized of the modern pocket gophers. The subgenus
+_Pappogeomys_ is regarded as the ancestral lineage, and the subgenus
+_Cratogeomys_ is regarded as an early offshoot, probably in the early
+Pleistocene, that became progressively more specialized in the course
+of its subsequent evolution. In the same period of time, the subgenus
+_Pappogeomys_ changed little. It is known only from late Pliocene
+fragments and from the living species. The ancestral morphotype is
+preserved in _Pappogeomys_. Primitive characters are: (1) Small size;
+(2) skull generalized and smoothly rounded; (3) temporal ridges
+separate (not uniting into a sagittal crest); (4) enamel plates
+retained on both anterior and posterior walls of M1 and M2; (5) M3
+bilophate, its posterior loph short. Basic specializations are few and
+include loss of the inner groove from the anterior face of the upper
+incisor; anteroposterior compression of the lateral re-entrant folds
+of the premolars; and loss of enamel from the posterior wall of P4.
+All three features have been perpetuated in the advanced subgenus
+_Cratogeomys_, suggesting that they were already developed in the
+early evolution of the subgenus _Pappogeomys_ before _Cratogeomys_
+diverged. Agreement with _Geomys_ is demonstrated by the lack of
+enamel on the posterior wall of P4 (see Fig. 9) and by retention of
+the posterior enamel plate on M1 and M2. In _Pappogeomys (Pappogeomys)
+alcorni_ the enamel from the posterior face of M1 has been lost from
+all but the lingual fourth or so of the posterior wall (Fig. 9E).
+Reduction of enamel in M1 provides an example of parallelism with the
+more advanced subgenus _Cratogeomys_, discussed below.
+
+There is no record as yet of the early evolution of the subgenus
+_Cratogeomys_. The features that characterize the subgenus were
+already well developed in the first known fossils which are from
+Wisconsin deposits of the late Pleistocene. _Cratogeomys_ is not a
+homogenous assemblage; instead it is composed of two groups of living
+species, the generalized _castanops_ group and the specialized
+_gymnurus_ group. The _castanops_ group may be survivors of the
+ancestral lineage that diverged in two different stages in the
+phyletic development of the main line. Even so, the _castanops_ group
+has acquired its peculiar specializations. Indeed, _P. merriami_ of
+the _castanops_ group differs from the hypothetical stem more than
+does _P. castanops_. Judging from the structure of the living species
+of the subgenus _Cratogeomys_ and from the primitive subgenus
+_Pappogeomys_, the subgenus _Cratogeomys_ featured five major trends:
+(1) Increase in size; (2) formation of sagittal crest by union of the
+temporal impressions; (3) increase in rugosity and angularity of the
+skull; (4) progressive development of platycephalic specializations,
+including the elongation of the angular process of the mandible; (5)
+complete loss of enamel plates from the posterior wall of M1 and M2.
+Each trend is thought to be adaptive.
+
+    [Illustration: FIG. 9. Molariform dentitions of the Tribe Geomyini.
+       Drawings illustrating enamel patterns characteristic of _Geomys_
+       and _Pappogeomys_ (including the subgenera _Pappogeomys_ and
+       _Cratogeomys_). × 5.
+
+    A and B. _Geomys bursarius bursarius_, adult female, No. 46275 (KU),
+             Elk River, Sherborne Co., Minnesota. Left upper (A), P4-M3;
+             right lower (B), p4-m3.
+
+    C and D. Subgenus _Pappogeomys_. _Pappogeomys bulleri albinasus_,
+             adult female, No. 31002 (KU), W side La Venta, 13 mi. W and
+             4 mi. N Guadalajara, Jalisco. Left upper (C), P4-M3; right
+             lower (D), p4-m3.
+
+    E and F. Subgenus _Pappogeomys_. _Pappogeomys alcorni_, adult
+             female, No. 31051 (KU), holotype, 4 mi. W Mazamitla, 6600
+             ft., Jalisco. Left upper (E), P4-M3; right lower (F),
+             p4-m3.
+
+    G and H. Subgenus _Cratogeomys_. _Pappogeomys gymnurus tellus_,
+             adult female, No. 31051 (KU), 1 mi. NE Tala, 4400 ft.,
+             Jalisco. Left upper (G), P4-M3; right lower (H), p4-m3.
+    ]
+
+Loss of enamel is a trend common to all living genera of the tribe
+Geomyini, but the greatest loss has occurred in _Cratogeomys_. It has
+lost the plates on the posterior walls of M1 and M2 (Fig. 9G). If the
+lateral plates of M3 are considered as one functional plate and the
+lateral plates on either side of P4 together as two transverse plates,
+then, the transverse cutting blades in _Cratogeomys_ number seven in
+the upper and seven in the lower cheek teeth compared with 10 in the
+upper and seven in the lower in the primitive morphotype. Indeed, in
+some species of the subgenus, one or both of the lateral plates on M3
+is also lost, usually in old age, resulting in even greater reduction
+of enamel. Loss of enamel from the posterior walls of the upper
+molars may be associated with changes in the mechanics of mastication
+from anteroposterior planing to anterotransverse shearing, as
+discussed elsewhere. Merriam (1895:95-96) argues convincingly that the
+posterior cutting blades of the upper molars would hinder efficient
+shearing action of the teeth; hence, selection would favor their
+reduction and eventual loss. Changes in the shape of the skull also
+seem to be correlated with the shift from a planing to a shearing type
+of mastication. More efficient shearing action, which depends upon
+lateral movement of the jaw, can be developed if the functional
+muscles insert farther laterally than is possible in the generalized
+type of skull. Therefore, platycephalic specializations involved
+lateral expansion of the braincase and mandible. Pronounced lateral
+expansion has been developed only in the _gymnurus_ group of species,
+suggesting that the dental specializations evolved earlier in the
+evolution of the subgenus than did the platycephalic specializations
+of the skull, and that the _castanops_ group separated from the
+_gymnurus_ group before the common ancestor had developed the more
+extreme trends in platycephaly. It is interesting to note that the
+subtriangular M3 (Fig. 9G) postulated for the ancestral morphotype and
+that characterizes the subgenus _Pappogeomys_ is retained also in the
+_gymnurus_ group.
+
+
+
+
+LITERATURE CITED
+
+
+ALSTON, E. R.
+
+
+    1876.  On the classification of the order Glires. Proc. Zool. Soc.
+           London, 1876:61-98, 1 pl., June.
+
+
+ALVAREZ, T.
+
+
+    1964.  Nota sobre restos oseos de mamiferos del Reciente, encontrados
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+
+    1965.  Catálago Paleomastozoológico Mexicano. Publ. Inst. Nac. Antro,
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+
+AXLEROD, D. I.
+
+
+    1950.  Studies in late Tertiary paleobotany. Carnegie Inst.
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+
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+BADER, R. S., and TECHTER, D.
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+
+BAIRD, S. F.
+
+
+    1858.  Mammals. Part I. General report upon the zoology of the
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+
+
+BARBOUR, H., and SCHULTZ, C. B.
+
+
+    1937.  An early Pleistocene fauna from Nebraska. Amer. Mus. Nat.
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+BERRY, E. W.
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+BLACK, C. C.
+
+
+    1961.  Rodents and lagomorphs from the Miocene Fort Logan and Deep
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+BRANDT, J. F.
+
+
+    1855.  Beiträge zur mähern Kenntiss der Säugethiere Russlands. Acad.
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+
+
+BROWN, B.
+
+
+    1908.  The Conard fissure, a Pleistocene bone deposit in northern
+           Arkansas with description of two genera & 20 new species of
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+
+    1912.  Brachyostracon, a new genus of glyptodont from Mexico. Bull.
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+
+CHANEY, R. W.
+
+
+    1947.  Tertiary centers and migration routes. Ecol. Monog.,
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+
+
+COOK, H. J., and COOK, M. C.
+
+
+    1933.  Faunal lists of the Tertiary Vertebrata of Nebraska and
+           adjacent areas. Nebraska Geol. Surv., Paper No. 5:49.
+
+
+COPE, E. D.
+
+
+    1878.  Description of new extinct Vertebrata from the upper Tertiary
+           and Dakota formations. Bull. U. S. Geol. and Geog. Surv.,
+           4:379-396.
+
+    1884.  The White River and John Day faunae, pp. 759-1002, pl. 64,
+           figs. 1-9. _In_ The Vertebrata of the Tertiary formations of
+           the west. Book I. Part second. Rept. U. S. Geol. Surv. Terr.
+           (F. V. Hayden), vol. 3.
+
+    1889.  The vertebrate fauna of the Equus beds. Amer. Nat.,
+           23:160-165.
+
+
+COUES, E.
+
+
+    1877.  Monographs of North American Rodentia. No. 8, Saccomyinae, pp.
+           483-542, and No. 10, Geomyinae, pp. 601-629, pl. 7, August.
+
+
+DALQUEST, W. W.
+
+
+    1962a. A record of the giant bison (Bison latifrons) from Cooke
+           County, Texas. Texas Jour. Sci., 13:41-44, March.
+
+    1962b. The Good Creek Formation of Texas, and its fauna. Jour.
+           Paleont., 36:568-582.
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+
+
+_Transmitted May 29, 1967._
+
+[]
+
+
+      *      *      *      *      *
+
+
+Transcriber's note:
+
+All obvious typographical errors were corrected. Minor changes were
+made to standardize the text to match the most prevalent form used.
+
+
+Typographical Corrections
+
+  Page Correction
+  ==== ============
+   477 cumberlandicus => cumberlandius
+   535 breath => breadth
+
+
+
+
+
+
+End of the Project Gutenberg EBook of Evolution and Classification of the
+Pocket Gophers of the Subfamily Geomyinae, by Robert J. Russell
+
+*** END OF THE PROJECT GUTENBERG EBOOK 40282 ***