North American Recent Soft-Shelled Turtles (Family Trionychidae)

Chapter I: Part 1

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UNIVERSITY OF KANSAS PUBLICATIONS

MUSEUM OF NATURAL HISTORY

Volume 13, No. 10, pp. 429-611, pls. 31-54, 24 figs.

February 16, 1962

North American Recent Soft-shelled Turtles

(Family Trionychidae)

ROBERT G. WEBB

UNIVERSITY OF KANSAS

LAWRENCE

1962

UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman, Henry S. Fitch, Robert W. Wilson

Volume 13, No. 10, pp. 429-611, pls. 31-54, 24 figs.

Published February 16, 1962

UNIVERSITY OF KANSAS

Lawrence, Kansas

PRINTED IN

THE STATE PRINTING PLANT

TOPEKA, KANSAS

1962

28-7818

North American Recent Soft-shelled Turtles

(Family Trionychidae)

ROBERT G. WEBB

CONTENTS

PAGE

CONTENTS 431

INTRODUCTION 433
Collecting Methods 434
Materials and Procedure 437
Acknowledgments 439

TAXONOMY 439
Family Trionychidae Bell, 1828 439
Genus _Trionyx_ Geoffroy, 1809 443
Variation 445
Secondary Sexual Variation 446
Ontogenetic Variation 449
Geographic Variation 453
Character Analysis 460
Composition of the Genus _Trionyx_ in North America 476
Artificial Key to North American Species and Subspecies
of the Genus _Trionyx_ 476
Systematic Account of Species and Subspecies 479
_Trionyx ferox_ 479
_Trionyx spinifer_ 486
_Trionyx spinifer spinifer_ 489
_Trionyx spinifer hartwegi_ 497
_Trionyx spinifer asper_ 502
_Trionyx spinifer emoryi_ 510
_Trionyx spinifer guadalupensis_ 517
_Trionyx spinifer pallidus_ 522
_Trionyx ater_ 528
_Trionyx muticus_ 531
_Trionyx muticus muticus_ 534
_Trionyx muticus calvatus_ 539

NATURAL HISTORY 541
Habitat 541
Daily and Seasonal Activity 547
Diurnal Habits 547
Behavior Adaptations 549
Movement 552
Nocturnal Habits 553
Seasonal Occurrence 553
Food Habits 555
Reproduction 558
Size of Males at Sexual Maturity 558
Size of Females at Sexual Maturity 560
Sexual Activity 563
Deposition of Eggs 565
Reproductive Potential 568
Eggs 572
Incubation and Hatching 573
Age and Growth 574
Mortality 576
Parasites 576
Economic Importance 577

EVOLUTIONARY HISTORY 578
Distribution 578
Relationships 579
Fossils 582
Phylogeny 585
The Importance of the Study of Turtle Populations in
Relation to the History of River Systems 588

SUMMARY 590

LITERATURE CITED 594

INTRODUCTION

Is it true that the greater the degree of resemblance between two populations the shorter the time the two have been spatially isolated? Are aquatic environments more stable than terrestrial environments? These questions occurred to me while I was collecting turtles from river systems of the Gulf Coast. As a general rule, each kind of turtle seemed to occur throughout one continuous river system or large tributary, and with no barriers to dispersal therein and with the lapse of enough time for a population to reach its limits of dispersal, the question arose, "Where do subspecies and zones of intergradation occur?" It seemed logical to think that each isolated and continuous aquatic environment would not contain more than one subspecies of the same species. In terrestrial environments subspecies and transitions between them were recognizable. Terrestrial habitats were continuous for longer distances than the isolated, aquatic habitats. But, different species of turtles prefer different kinds of aquatic habitats. Also, barriers occur in large drainage systems, such as the Mississippi, where, in general, the western tributaries are sluggish, turbid and shallow, and the eastern tributaries are fast-flowing, clear and deep. But in young, relatively small, river systems that do not traverse radically different physiographic regions, and that show no gross ecological differences, habitats or microhabitats that do exist probably are only partial barriers and seem not to prevent the dispersal of most kinds of aquatic turtles. Consequently, it seemed that study of the degree of difference between closely related populations of turtles that occurred in one drainage system, or in adjacent drainage systems would indicate the length of time, respectively, that the drainage system had been continuous or the length of time that two or more systems had been isolated from one another.

Rivers or series of river systems having endemic kinds of turtles or having the most kinds of turtles that are different from those in adjacent rivers may be the oldest geologically, or may have been isolated the longest. Knowledge of the kinds of turtles and their relationships and distribution could indicate chronological changes in aquatic habitats. Of course, modifying factors such as differences between populations of turtles in rates of evolutionary change, degrees of vagility, rates of dispersal, and overland migrations need to be taken into account.

My accumulation of data on soft-shelled turtles was begun in the early nineteen-fifties. Although American softshells have been discussed in a revisionary manner by Agassiz (1857), Siebenrock (1924), Stejneger (1944) and Neill (1951), the relationships of all the component populations have not hitherto been appreciated. The present account attempts to combine in one publication what is known concerning the taxonomy, geographic distribution, life history, and relationships of the Recent American species and subspecies of the genus _Trionyx_.

Collecting Methods

Nocturnal collecting, by hand, from a boat that was nosed among brush piles along the shore line of rivers (Chaney and Smith, 1950:323) in the early 1950's on rivers of the Gulf Coast drainage east of Texas yielded many turtles of the genus _Graptemys_ but few softshells. Chaney and Smith (_loc. cit._) reported only one softshell among 336 turtles taken in 21 collecting hours on July 5, 6 and 7 on the Sabine River; Cagle and Chaney (1950:385), however, recorded 11.6 per cent softshells of 208 turtles (collecting time not stated) taken on the Caddo Lake Spillway in Louisiana. Using hoop-nets is probably the most efficient method for collecting softshells considering the time and effort involved, and is the chief method I have used. Lagler (1943a:24) mentioned the use of watermelon rind as an effective bait. Kenneth Shain (field notes) trapped _T. spinifer emoryi_ in hoop-nets baited with bread. I have used chopped fresh fish with most success; canned sardines have also been satisfactory. These baits seem to be more successful for trapping _spinifer_ than they are for _muticus_. Hoop-nets were used to trap turtles in Lake Texoma, Oklahoma, from June 14 to July 2, 1954. The number of traps (usually four, rarely five) and trapping success varied with location. Of 156 turtles, 19 (12%) were _T. spinifer_ and one was _T. muticus_.

Trotlines and set lines frequently catch softshells; sport fishermen often complain of catching these turtles on hook and line. Live worms, soft-bodied insects, small crawfish, minnows, small pieces of fish and other kinds of meat are adequate bait. Capture depends on the skill of attachment of the bait and the size of hook used. In my experience, softshells (mostly _spinifer_) were taken on trotlines that were set in lakes or the slower-moving parts of rivers a few inches below the surface. I have records of only two _muticus_ taken on trotlines. Goin (1948:304) stated that commercial fishermen catch softshells on trotlines set for catfish on the bottom of river beds. Evermann and Clark (1920:595) found softshells to be caught more often than any other kind of turtle in traps, on set lines, and by anglers in Lake Maxinkuckee, Indiana. Some residents of the South tell of so placing baits that turtles are lured to tread water against an object set with recurved hooks upon which the webbing of the forelimbs are impaled.

Individuals of _muticus_ and _spinifer_ frequently bury themselves in sand in shallow water and can be collected by hand by noting swirls or disturbances on the bottom caused by a turtle withdrawing its head (Conant, 1951:156, 159). Professional turtle collectors take them by "noodeling" (Conant, _op. cit._:160); Lagler (1943a:22) elaborated on the method of "noodling." P. W. Smith (1947:39) remarked that 20 or more softshells were taken "within a few hours by probing sand bars at the water edge" near Charleston, Illinois. From a distance I observed an individual of _T. s. asper_ bury itself in shallow water on the Escambia River, Florida. Small individuals of _muticus_ have been taken by hand along the shore of Lake Texoma. Along the Flint River near Bainbridge, Georgia, two hatchlings that were buried in sand in shallow water emerged at my approach and scurried a few inches, then buried themselves again. Larger turtles seem to be more wary. One that was disturbed, emerged from the sand and swam toward deep water.

In clear water, water-goggling may be effective in securing softshells. Marchand (_in_ Carr, 1952:417-18) mentioned that softshells (_ferox_) can be found buried in deep water with only the heads visible; the turtles are not easily frightened under water and may be captured by grasping their necks. A similar technique described by Allen and Neill (1950:3) resulted in the capture of trionychid turtles. In clear water of the White River, Arkansas, I collected a few softshells by hand as they lay on the bottom.

In shallow-water areas of large rivers, lakes and tributaries, seining often procures softshells. Methods used in fisheries investigations such as the application of rotenone and electric shockers, and even dynamiting, sometimes yield soft-shelled turtles. Carr (1952:419) wrote that numbers of _ferox_ were incapacitated by rotenone in Florida lakes, although no other species of turtle was affected. I captured a snapping turtle (_Chelydra serpentina_) that was immobilized by the current from an electric shocker in a small, alga-choked tributary of Cache Creek, Comanche County, Oklahoma; presumably turtles must come in close contact with the electrodes to be affected (see discussion by Gunning and Lewis, 1957:52).

The effectiveness of gill nets in trapping turtles is indicated by information kindly supplied by Mr. Alfred Houser on gill-net operations from July through December, 1952, under the direction of Mr. "Bud" Oldham, a commercial fisherman. The 4-inch mesh nets were in Lake Texoma at the mouth of Briar Creek, two miles south of Powell, Marshall County, Oklahoma, in 25 to 30 feet of water. Eighty to 90 per cent of the turtles secured were softshells; more were taken near shoreline than away from shore even though the depth was about the same. An average of only one turtle every four days was taken in July and August when the turtles presumably are most active (Table 1). One gill-net day is equivalent to one gill net, 200 yards long, operated for 24 hours.

TABLE 1. The Abundance of Turtles as Revealed by Gill-net
Operations in Lake Texoma, 1952.

===========+==========+===========+===============
| Gill-net | Number of | Gill-net days
MONTH | days | turtles | per turtle
-----------+----------+-----------+---------------
July | 835 | 213 | 3.9
August | 816 | 199 | 4.6
September | 743 | 42 | 17.7
October | 1661 | 82 | 20.3
November | 1322 | 48 | 27.5
December | 864 | 5 | 172.8
-----------+----------+-----------+---------------

Dr. Virgil Dowell, while making fishery studies two miles east of Willis, Marshall County, Oklahoma, caught, on the average, 1.5 turtles per day. Of 75 turtles collected from July 1 through October 18, 1953, 66 were _Trionyx_ (_spinifer_ and _muticus_), five were _Graptemys_ and four were _Pseudemys scripta_. No more than two gill nets were used simultaneously. The nets were moved from time to time and varied in dimensions, but those used most of the time were 200 feet long and eight feet deep with a 3-inch mesh.

The few captures by Houser probably resulted from long-continued trapping in one place; the gill nets were not moved in the entire six-month period or for some time previously. Breckenridge (1955:6) commented on the sedentary nature of _spinifer_ (in Minnesota) and quoted a professional turtle trapper as stating that "after a section of a river has been trapped heavily for softshells, little success can be expected in that area for as much as three or four years thereafter." Both Houser's and Dowell's data indicate a higher percentage of soft-shelled turtles collected than any other species. The number caught probably depends, at least partly, on the food habits of the species and is influenced by the enmeshed fish, which, serving as a food source, attract the turtles.

Materials and Procedures

In the course of this study I examined 1849 soft-shelled turtles, including some incomplete alcoholic or dried specimens, such as those represented only by skulls or by other osteological material. Material was examined from each of the collections named below (except KKA), and these are mentioned in the text by the following abbreviations:

AMNH American Museum of Natural History
ANSP Academy of Natural Sciences, Philadelphia
BCB Bryce C. Brown, private collection, Baylor University
CM Carnegie Museum
CNHM Chicago Natural History Museum
INHS Illinois Natural History Survey, University of Illinois
KKA Kraig K. Adler, private collection, data in letter dated January 8, 1960
KU Museum of Natural History, The University of Kansas
LSU Louisiana State University
MCZ Museum of Comparative Zoology, Harvard College
MSU The Museum, Michigan State University
NHB Naturhistorisches Museum Basel, Switzerland
OU University of Oklahoma Museum, Division of Zoology
SM Strecker Museum, Baylor University
TCWC Texas Cooperative Wildlife Collection, Texas Agricultural and Mechanical College
TNHC Texas Natural History Collection, The University of Texas
TTC Texas Technological College
TU Tulane University
UA University of Alabama
UI Museum of Natural History, The University of Illinois
UMMZ Museum of Zoology, The University of Michigan
USNM United States National Museum
WEB William E. Brode, private collection, Mississippi Southern College
WTN Wilfred T. Neill, private collection

External measurements (listed under the section, "Variation") were taken by the writer by means of a Vernier caliper or a steel tape. Measurements of the skulls are in millimeters and tenths as taken by the writer with dial calipers. Partial wrinkling of the carapace at the edges of some specimens causes some error in measurements; consequently, length of plastron is used as the measurement of reference.

Scattergrams based on external measurements were constructed. Some demonstrate considerable ontogenetic variation. An inspection of the scattergrams indicated regressions essentially linear in nature, but sometimes occasioned an arbitrary separation of samples into size groups to show ontogenetic variation; no secondary sexual differences could be discerned. Several ratios were developed from the measurements. The data correspond to the regression model 1A in "Statistical Methods" (Snedecor, 1956, sec. 6.13); consequently, the sample ratios indicate the slope of regression and are useful in comparisons. Sample-means and their estimated standard errors are compared graphically to show general trends in proportional characters. Comparisons of means and standard errors indicate statistical significance between populations if the sample-means plus or minus twice their standard errors do not overlap, but this method of comparison is valid only when comparing two samples (Pimentel, 1959:100).

In the section on "Variation," general features applicable to all kinds of soft-shelled turtles are discussed under the following headings: secondary sexual, ontogenetic, and geographic; individual variation is mentioned in accounts of species and subspecies. In the section "Character Analysis" external and osteological characters having taxonomic significance are discussed.

Vernacular names follow, as closely as possible, those recommended by the Committee on Herpetological Common Names (1956). The synonymy of each monotypic species or subspecies begins with the name as given in the original description. The second entry is the name-combination herein applied to the taxon. Other entries are first usages, in chronological order, of other names (synonyms) that have been applied to the taxon in question. Next, the type is briefly discussed followed by the "Range" defined in general geographic terms, and, when appropriate, in terms of river drainage systems. "Diagnosis" includes a combination of characters that facilitates quick identification. In polytypic species, the diagnosis of a subspecies is designed only to distinguish it from other subspecies of that species. The comments included under the subsection entitled "Description" pertain to individuals from an area where the taxon is most clearly differentiated. Because osteological characters are significant only at the specific level, they appear under the accounts of each species (excluding _ater_). Proportional characters as given in the "Diagnosis" are only in general terms; more specific data are set forth in the subsection, "Description" or in the various text figures, mostly in the section on "Variation," page 445. Proportions pertaining to the species _muticus_ were derived only from the nominal subspecies, and appear under the account of the species. A subsection "Variation" under the accounts of some subspecies includes information concerning principally individual variation and coloration; because color is not considered to be of major taxonomic importance, color terms are used without reference to any standard color guide. The subsection "Remarks" follows the section on "Comparisons," and may include comments on nomenclature, intergradation and other information related to the distribution or taxonomy of the subspecies.

The probable geographic range of each species and subspecies is shown on one of the maps. Locality records of specimens that I have examined are shown by solid circles. Additional records of occurrence (published records or specimens otherwise not seen) are shown by hollow circles. Localities only a short distance apart share the same circle.

Under the subsection "Specimens examined," a number in parentheses following a museum number indicates the number of specimens referable to that museum number. All localities of specimens examined are indicated on one of the maps. The list of specimens is arranged alphabetically by states (Canadian provinces precede states of the United States under the account of _T. spinifer spinifer_, and Mexican states follow those of the United States under _T. s. emoryi_), alphabetically by counties, and within a county alphabetically by abbreviations of museums; then, museum catalogue numbers are arranged consecutively. Records in the literature are not included if they refer to the same locality from which at least one specimen has been examined, or refer to a less restricted locality that includes the area from which at least one specimen has been examined. Localities within a county are arranged alphabetically by author; the appropriate reference may follow several localities.

All generic, specific and subspecific names (but not all the different kinds of name-combinations) that have been applied to American soft-shelled turtles are listed in a subsection entitled "Synonymy" under the heading "Genus Trionyx Geoffroy, 1809."

Acknowledgments

Completion of this study has been made possible only by the co-operation of those persons in charge of the collections listed above and I am grateful to them for the privilege of examining specimens. Also I wish to thank Dr. E. Raymond Hall for the facilities afforded by the Museum of Natural History at the University of Kansas, as well as for editorial assistance in the preparation of the manuscript, and especially Dr. Henry S. Fitch under whose guidance this research was carried out.

In addition to various staff members, graduate students, and individuals whose help is acknowledged at appropriate places in the text, Dr. Rollin H. Baker, Dr. Fred R. Cagle, Mr. J. Keever Greer, Dr. A. Byron Leonard, Dr. Carl D. Riggs, and Dr. Edward H. Taylor deserve especial mention for aid extended in the course of this study. I am indebted to Mr. J. C. Battersby, British Museum (Natural History), London, for information concerning the type of _Trionyx ferox_, to Dr. Jean Guibé, Museum d'Histoire Naturelle, Paris, for information concerning the types of _Trionyx muticus_, _T. spinifer_ and _T. carinatus_, and photographs of the types of _T. muticus_, _T. spinifer_ and _T. ocellatus_, and to Dr. Lothar Forcart of the Naturhistorisches Museum, Basel, Switzerland, for information pertaining to a published record of _T. muticus_.

The maps and figures are the work of Miss Lucy Jean Remple and Mrs. Lorna Cordonnier, University of Kansas. Dr. John M. Legler, University of Utah, prepared most of the photographs on plates 1-20; photographs as mentioned in the preceding paragraph were received from Dr. Guibé, one was provided through the co-operation of Roger Conant and Isabelle Hunt Conant, another was furnished by Mr. J. Keever Greer, and the others were taken by me. Field work was financed in part by funds provided by the Sigma Xi-RESA Research Fund.

TAXONOMY

Family Trionychidae Bell, 1828

Recent soft-shelled turtles comprise a well-defined assemblage of the family Trionychidae. Although the scope of this study does not involve an assay of the relationships of the soft-shelled turtles of the Old World, a brief résumé that includes some of the salient characteristics of the family is included.

_Diagnosis._--Articulation between last cervical and first dorsal vertebrae by zygopophyses only; preplastra separated from hyoplastra by /\-shaped epiplastron, entoplastron absent (Williams and McDowell, 1952:263-75); marginal bones absent or forming an incomplete series, not connected with ribs that extend beyond pleural plates; claws on only three inner digits; fourth digit having four or more phalanges; plastron united to carapace by ligamentous tissue (Smith, 1931:147).

_General characters._--Size large, "... some attaining probably 5 feet in length of carapace" (Boulenger, 1890:10); body depressed; carapace and plastron lacking horny epidermal shields, covered instead with soft skin; snout ending in fleshy, tubate proboscis; jaws concealed by fleshy lips; tail short; digits well-webbed; cervical vertebrae opisthocoelous (eighth having double articulation in front); neck elongate, cervical region equaling or exceeding length of dorsal vertebral column; head and neck completely retractile, bending by means of sigmoid curve in vertical plane; ear hidden; skull elongate, having three posterior projections (median one produced by supraoccipital and two lateral projections formed chiefly by squamosals); temporal region emarginate posteriorly, forming wide shallow fossa; premaxillae fused; an intermaxillary foramen; pterygoids separated by basisphenoid that contacts palatines; vomer, if present, not separating palatines; pelvis not fused to carapace and plastron; plastron reduced, a median vacuity usually present; plastral bones developing sculpturing with increase in size, forming four to seven so-called plastral callosities; carapace with or without prenuchal bone; nuchal overlapping or overlapped by first pleural; neurals in continuous series or interrupted by pleurals; bony plates of carapace sculptured; mandible having well-developed coronoid bone; cutaneous femoral valves that conceal hind limbs present or absent; two or three pairs of scent glands; cloacal bursae absent (Smith and James, 1958:89); forelimbs having antebrachial scalation; body of hyoid apparatus formed of two or three pairs of bones; penis broad, expanded and pentifid, sulcus spermaticus quadrifid having branches in each of four lateral projections (Hoffman, 1890:298, pl. 47, fig. 2); aquatic, principally in fresh water; mainly carnivorous; flesh of many species eaten. (See Boulenger, 1889:237-41; Loveridge and Williams, 1957:412; Romer, 1956:513; Smith, _op. cit._:147-54).

_Recent distribution_ (Figure 1).--North America, from extreme southeastern Canada and eastern United States west to Rocky Mountains and south to northern México; introduced in southwestern United States (Conant, 1958:69-73). Africa, from Egypt and Senegal south to Angola and Zambesi River drainage (Loveridge and Williams, _op. cit._:412-68); occurrence of _Trionyx triunguis_ in Syria (Boulenger, _op. cit._:255) and coastal streams of Palestine (Schmidt and Inger, 1957:36) considered accidental by Flower (1933:753-54). Southwestern Asia (Tigris and Euphrates River drainage) in eastern Turkey, Syria, Iraq and northeastern Israel (Mertens and Wermuth, 1955:388). Southeastern Asia, from Pakistan and India (Indus River drainage) and Manchuria and adjacent Siberia (Amur River drainage) to Ceylon, Japan, Formosa, Hainan, Luzon, Sumatra, Java, Borneo, Timor and southeastern New Guinea (De Rooij, 1915:325-32; Okada, 1938:108; Pope, 1935:60-64; Smith, 1931:158-79; Stejneger, 1907:514-532; Taylor, 1920:141).

_Trionyx cartilagineus_ is questionably recorded from the Moluccas (De Rooij, _op. cit._:330). _T. sinensis_ has been introduced on Kauai Island, Hawaiian Islands (Brock, 1947:142; Oliver and Shaw, 1953:83), one of the Bonin Islands (Okada, 1930:187-94), and probably Timor (De Rooij, _op. cit._:331). All insular records east of Borneo and Java are probably the result of introductions, except perhaps those of _Pelochelys_ on Luzon and New Guinea (Darlington, 1957:210).

_Recent genera._--According to Mertens and Wermuth (1955:387-95), there are 21 species belonging to six genera as follows:

_Chitra_ Gray, 1844 (1)
_Cyclanorbis_ Gray, 1854 (2)
_Cycloderma_ Peters, 1854 (2)
_Lissemys_ Smith, 1931 (1)
_Pelochelys_ Gray, 1864 (1)
_Trionyx_ Geoffroy, 1809 (14)

_Dogania_ is considered a synonym of _Trionyx_ (Loveridge and Williams, _op. cit._:422).

_Geologic range._--Lower Cretaceous (possibly Upper Jurassic) to Recent of Asia; Upper Cretaceous to Recent of North America; Paleocene (Upper Jurassic, assuming _Trionyx primoevus_ is a trionychid) to Pleistocene of Europe; Lower Miocene to Recent of Africa; Pleistocene to Recent in East Indies (Loveridge and Williams, _op. cit._:412; Romer, 1945:594); questionable trionychid fragments from Pleistocene of Australia (Darlington, _loc. cit._). $/

_Remarks._--The genera _Lissemys_, _Cyclanorbis_ and _Cycloderma_ are distinguished from _Pelochelys_, _Chitra_ and _Trionyx_ by several characters (Loveridge and Williams, _op. cit._:414). The recognition of two groups of genera caused Deraniyagala (1939:290) to erect two families, Cyclanorbidae and Trionychidae. An appraisal of fossils prompted Hummel (1929:768) to propose two corresponding subfamilies, Cyclanorbinae and Trionychinae. Williams (1950:554) considered the two groups as subfamilies (Lissemydinae and Trionychinae).

Baur (1887:97) regarded the Trionychidae as constituting a separate suborder distinct from the rest of the living turtles. Later (1891), however, he pointed out the resemblances of the Trionychidae and Carettochelyidae (having one living genus in New Guinea), and the cryptodiran affinities of _Carettochelys_. Bergounioux (1932:1408) mentioned the close resemblance of the Carettochelyidae to _Trionyx_ but considered the former as having pleurodiran affinities, a view adopted by Deraniyagala (_loc. cit._). Most students now consider the two families to be closely related, and conceive of both as members of the suborder Cryptodira (Hummel, 1929:768; Williams, _loc. cit._; Mertens and Wermuth, 1955).

The oldest trionychid fossil, _Trionyx primoevus_, is from marine deposits of the Upper Jurassic (Kiméridgien) from "Cap de la Hève," and its characters do not indicate the kind of cryptodiran ancestor from which the family arose (Bergounioux, _op. cit._:1409; 1937:188). Lane (1910:350) found that the entoplastron (= epiplastron) was paired in embryos of _Trionyx_ and regarded that genus as the most primitive of the order; he also mentioned Wiedersheim's report of rudiments of teeth in embryos of _Trionyx_. Baur (1891:637-38) thought that the family arose directly from the Amphichelydia, that the ancestors of the Trionychidae closely resembled _Carettochelys_ in the structure of the carapace and plastron, and that a progressive reduction in ossification of those structures occurred. Nopcsa (1926:654) also wrote that the family originated from ancestors having a well-developed plastron; he maintained that the progressive reduction in ossification of the plastron was a specialization for aquatic life, and that the more primitive trionychids had the best developed bones and callosities. Hummel (1929:772) also thought that there had been a progressive reduction in ossification. Bergounioux (1932:1408; 1936:1088, 1952:2304), on the contrary, thought that there had been a progressive increase in ossification of the marginal bones in both families as well as of the plastron (1936:1088; 1937:190). Zangerl's study of the shell elements of turtles (1939:393) indicated that _Trionyx_ was highly specialized in having a well-developed epithecal armor (sculptured callosities, neurals and costals), and that it occurred in most aquatic turtles; the development in soft-shells suggested that members of the family had maintained an aquatic mode of life over a long period of geologic time, a view supported by Deraniyagala (1930:1066). Of interest are Stunkard's remarks (1930:214-18) concerning several _Trionyx spinifer_ that were obtained from a commercial supply house and found to be infested with pronocephalid trematodes (_Opisthoporus_ [= _Teloporia_] _aspidonectes_). The closest relatives of that trematode (also recorded from _T. ferox_) live in marine turtles. Possibly, a Mesozoic ancestor of marine and essentially fresh-water soft-shelled turtles harboured ancestors of these trematodes, but possibly the parasites may have transferred relatively recently to their present hosts. Bergounioux (1937:190) judged the Trionychidae to be an ancient group of marine origin. Hummel (1929:770) wrote that the Trionychidae originated in east Asia (the region of most differentiation) in humid climates.

Baur (1891:634, 637) pointed out that the dorsal aspect of the skull of the closely related _Carettochelys_ resembles the skull of the Dermatemydidae, Staurotypidae and Kinosternidae; the close relationship of _Carettochelys_ and the Dermatemydidae is also mentioned by Bergounioux (1952:2304) and Hummel (1929:769). Hummel (_op. cit._:771) thought that the Carettochelyidae and "die Chelydroiden" had a common ancestor, and that (_op. cit._:772) the origin of the Trionychidae was older than those two groups. Dunn (1931:109) wrote that the Kinosternidae, Carettochelyidae and Dermatemydidae represented the same general ancestry. Williams (1950:552) has shown the resemblance of the cervical articulations in members of the Chelydridae (including Staurotypinae and Kinosterninae) and the Central American family Dermatemydidae. The consensus of opinion, then, is that the families Trionychidae, Carettochelyidae, Chelydridae and Dermatemydidae are relatively closely related.

Genus =Trionyx= Geoffroy, 1809

_Testudo_ Linnaeus (in part), Syst. Nat., Ed. 10, 1:197, 1758; type,
_Testudo graeca_ Linnaeus by subsequent designation (Fitzinger,
1843:29).

_Trionyx_ Geoffroy, Ann. Mus. Hist. Nat. Paris, 14:1, August, 1809;
type, _Trionyx aegyptiacus_ (= _Testudo triunguis_ Forskål) by
original designation.

_Apalone_ Rafinesque, Atlan. Jour., Friend of Knowledge,
Philadelphia, 1 (No. 2, Art. 12):64, Summer, 1832; type, _Apalone
hudsonica_ (= _Trionyx spiniferus_ Lesueur) by monotypy.

_Mesodeca_ Rafinesque, Atlan. Jour., Friend of Knowledge,
Philadelphia, 1 (No. 2, Art. 12):64, Summer, 1832; type _Mesodeca
bartrami_ (= _Testudo ferox_ Schneider) by monotypy.

_Aspidonectes_ Wagler, Naturl. Syst. Amphib., p. 134, 1830; type,
_Aspidonectes aegyptiacus_ Wagler (= _Testudo triunguis_ Forskål)
by subsequent designation (Fitzinger, 1843:30).

_Amyda_ Fitzinger, Ann. Wiener Mus. Naturg., 1:110, 120, 127, 1835;
type, _Amyda subplana_ Fitzinger by subsequent designation
(Fitzinger 1843:30).

_Gymnopus_ Duméril and Bibron, Erpét. Gén., 2:472, 1835; new
(substitute) name for _Aspidonectes_ Wagler.

_Pelodiscus_ Fitzinger, Ann. Wiener Mus. Naturg., 1:110, 120, 127,
1835; type, _Pelodiscus sinensis_ Fitzinger by subsequent
designation (Fitzinger, 1843:30).

_Platypeltis_ Fitzinger, Ann. Wiener Mus. Naturg., 1:109, 120, 127,
1835; type, _Platypeltis ferox_ by subsequent designation
(Fitzinger, 1843:30).

_Potamochelys_ Fitzinger, Syst. Rept., p. 30, 1843; type,
_Aspidonectes javanicus_ Wagler (= _Testudo cartilaginea_
Boddaert) by original designation.

_Tyrse_ Gray, Cat. Tort. Croc. Amphis. Brit. Mus., p. 48, 1844;
type, _Tyrse nilotica_ Gray (= _Testudo triunguis_ Forskål) by
tautonomy (_Tyrse_, a name for the Nile River).

_Callinia_ Gray, Proc. Zool. Soc. London, p. 222, 1869; new
(substitute) name for _Aspidonectes_ of Agassiz (1857:403); type,
_Callinia spicifera_ (mispelling for _spinifera_) Gray by
subsequent designation (Stejneger, 1907:514).

_Euamyda_ Stejneger, Bull. Mus. Comp. Zool., 94:7, 9, 12, 1944; new
(substitute) name for _Amyda mutica_ of Agassiz (1857:399); type,
_Amyda mutica_ Agassiz by monotypy.

_Type Species._--_Trionyx aegyptiacus_ (= _Testudo triunguis_ Forskål).

_Diagnosis._--Cutaneous femoral valves absent; width of postorbital arch of skull less than diameter of orbit; pterygoids usually not contacting opisthotics; carapace lacking prenuchal bone and marginal ossifications; nuchal bone lacking conspicuous ventral ridges; posterior margin of nuchal overlying first pair of pleurals; lateral parts of nuchal bone overlying second pair of ribs; neurals seven or eight, rarely six or nine; pleurals seven or eight pairs, posterior one or two pairs sometimes in contact medially; distinct suture usually present between hyoplastra and hypoplastra; most laterad prong of posteromedial process of hypoplastra inserted between bifid anterolateral process of xiphiplastra.

_Synonomy._--Geoffroy published a synopsis of the species he recognized (1809) prior to his formal description of the genus _Trionyx_ (1809a). Schweigger, nevertheless, probably was the first person to recognize the soft-shelled turtles as a distinct group, and he proposed for it the name _Amyda_ in an unpublished manuscript that he sent to Geoffroy. The latter author (1809a:15) relegated the name _Amyda_ to the synonomy of _Trionyx javanicus_ by means of the following entry: "_Amyda javanica._ Schweigger, dans un manuscript communique a l'Institut." Stejneger (1944:7) maintained that this publication of Schweigger's monotypic generic name clearly established its availability for the species congeneric with _Amyda javanica_ (= _Testudo cartilaginea_ Boddaert, 1770). Loveridge and Williams (1957:422) contend that this mere mention of the name _Amyda_ neither constitutes the proposal of a new name nor validates it, and that the first valid usage of the name _Amyda_ is that of Fitzinger (1835:120), who later (1843:30) designated the type species as _Amyda subplana_. The name _Amyda_ cannot date from _Oken_ (1816:348) as Volume 3 [Zoologie] of his Lehrbuch der Naturgeschichte published in 1815-1816 has been placed on the Official Index of Rejected and Invalid Works in Zoological Nomenclature with the Title No. 33; see Opinion 417 (Hemming, 1956).

There has been considerable debate as to whether Geoffroy did or did not designate a type species of the genus _Trionyx_ (1809a). Although not specifically designated as the type species, _Trionyx aegyptiacus_ (= _Testudo triunguis_ Forskål) is considered by Smith (1930:2), Schmidt (1953:108, footnote), and Loveridge and Williams (1957:422) to have been sufficiently indicated by Geoffroy as the type species. But Stejneger (1944:6), H. M. Smith (1947:122), Conant and Goin (1948:11), and Mertens and Wermuth (1955) maintained that Geoffroy did not adequately designate a type species, and that Fitzinger (1843:30) designated the type species as _Trionyx granosus_ (= _Lissemys punctata_), a synonym of Geoffroy's species, _coromandelicus_.

If Fitzinger's designation of a type species is accepted, the name _Trionyx_ is applicable to the forms herein referred to _Lissemys_, and _Amyda_ to the American forms. If Geoffroy's designation is accepted, the American forms are referable to _Trionyx_, and _Amyda_ is a synonym.

The preceding includes only those generic names (listed in chronological order) that have been applied to Recent American soft-shelled turtles. Generic synonyms of the genus _Trionyx_ applicable to Old World species are listed by Stejneger (1907:514), Smith (1931:165), and Loveridge and Williams (1957:420-21).

_Trionyx_ is the most widespread genus of the family; most of the species occur in southeastern Asia. All North American soft-shelled turtles belong to this genus.

For quick reference, all the specific and subspecific names proposed for soft-shelled turtles in North America are listed below in alphabetical order (left hand column) with their nomenclatural status as recognized in this paper. The synonyms are listed in the account of the appropriate species or subspecies, and are discussed under the subsection entitled "Remarks."

_agassizi_ _Trionyx spinifer asper_
_annulifer_ _Trionyx spinifer spinifer_
_argus_ _Trionyx spinifer spinifer_
_asper_ _Trionyx spinifer asper_
_ater_ _Trionyx ater_
_bartrami_ _Trionyx ferox_
_emoryi_ _Trionyx spinifer emoryi_
_calvatus_ _Trionyx muticus calvatus_
_ferox_ _Trionyx ferox_
_georgianus_ _Trionyx ferox_
_georgicus_ _Trionyx ferox_
_harlani_ _Trionyx ferox_
_hartwegi_ _Trionyx spinifer hartwegi_
_hudsonica_ _Trionyx spinifer spinifer_
_mollis_ _Trionyx ferox_
_microcephalus_ _Trionyx muticus muticus_
_muticus_ _Trionyx muticus muticus_
_nuchalis_ _Trionyx spinifer spinifer_
_ocellatus_ _Trionyx spinifer spinifer_
_olivaceus_ _Trionyx spinifer spinifer_
_spiniferus_ _Trionyx spinifer spinifer_

Variation

Aside from qualitative variations and comparisons of patterns of pigmentation the following external measurements (to the nearest millimeter) were used.

_Length of plastron_: Maximal straight-line measurement (midventrally), from the anteriormost region of the ventral surface to the posterior end of the plastron; this measurement includes an anterior cartilaginous part.

_Length of carapace_: Maximal, straight-line measurement (middorsally), from the nuchal region to the posteriormost region of the free edge of the carapace.

_Width of carapace_: Maximal, straight-line measurement between the lateral margins of the carapace.

_Plane of greatest width of carapace_: Maximal, straight-line measurement from the posteriormost region of the free edge of the carapace to a point on the middorsal line at the level or plane of the greatest width of the carapace; this measurement and the last two, of course, include the fringing cartilaginous parts of the dorsal bony carapace.

_Width of head_: Maximal measurement between the lateral margins of the head.

_Length of snout_: Measurement from tip of snout to interorbital region of least breadth.

_Diameter of ocellus_: Maximal outside diameter of largest (not conspicuously ovoid or oblong) ocellus on carapace.

The following ratios were developed from the measurements. Reference to these ratios will be made by the abbreviations within the parentheses: length of carapace/length of plastron (CL/PL); length of carapace/width of carapace (CL/CW); length of carapace/plane of width of carapace (CL/PCW); length of plastron/width of head (PL/HW); width of head/length of snout (HW/SL); diameter of ocellus/length of plastron (OD/PL).

Secondary Sexual Variation

_Size_

In many species of turtles, females are larger than males; the difference in size between the sexes is probably most pronounced in aquatic emydids. The ten largest individuals of each sex were selected to indicate the relative difference in size between the sexes of the three American species of _Trionyx_ (excluding _ater_, Table 2). Female soft-shelled turtles attain a larger size than males. _T. ferox_ is the largest species; _muticus_ is the smallest. The approximate maximal size of each sex and the difference in size between the sexes are more correctly expressed for _spinifer_ and _muticus_ than for _ferox_, because fewer specimens of _ferox_ were examined; presumably the approximate maximal size of males and females of _ferox_ is larger than is indicated in Table 2.

TABLE 2. Secondary Sexual Difference in Maximal Size of North
American Species of the Genus Trionyx (excluding ater) Based
on the Ten Largest Specimens of Each Sex of Each Species.
The Extremes Precede the Mean (in parentheses).

_Pattern_

Secondary sexual differences in pattern are probably more pronounced in soft-shelled turtles than in other species of turtles, except perhaps for the well-known melanism and concomitant obliteration of pattern acquired by some adult males of the _scripta_ section of the genus _Pseudemys_.

The difference in pattern between the sexes of American species varies with size of the individual and with the species and subspecies. The juvenal pattern of some individuals of _T. spinifer asper_ differs according to sex. In the other species and subspecies, there are no secondary sexual differences in the juvenal pattern. That pattern in females of all species and subspecies is partly or entirely obscured by a mottled and blotched pattern as growth proceeds. This mottled and blotched pattern is present on females not yet sexually mature, and is of contrasting lichenlike figures, and in other individuals is less contrasting and a more uniform coloration. The largest males of _T. spinifer_ retain a conspicuous juvenal pattern; in those of _muticus_ the pattern may be well-defined or partly modified and obscured, whereas in large males of _ferox_ the juvenal pattern is ill-defined or absent. No male normally acquires a contrasting mottled and blotched pattern on the carapace. The pattern on the carapace of many large individuals of _ferox_ is not distinctive as to sex.

On the dorsal surface of the soft parts of the body there is a contrasting pattern in adult males and hatchlings of some forms, but in most large females the pattern is usually reduced to a near-uniform coloration; the pattern on adult males of _ferox_ and _muticus_ is not contrasting and resembles that on large females.

_Coloration_

Because most specimens examined were preserved, the detection of secondary sexual differences in coloration was difficult. There is one difference in coloration between the sexes in the subspecies _T. s. emoryi_. Males from the Río Grande drainage, at least those from the Big Bend region of Texas, and southwestward in the Río Conchos into Chihuahua, México, are bright orange on the side of head (postlabial and postocular pale areas); an orange tinge also occurs in pale stripes on the snout, and pale orange blotches sometimes occur on the dorsal surfaces of limbs, especially the hind limbs. The coloration of these areas on females is pale yellow, lacking orange.

_Tuberculation_

In all subspecies of _spinifer_ the carapace of adult males is "sandpapery" owing to abundant, small, spiny tubercles distributed over its surface; all females lack spiny tubercles on the surface of the carapace.

_Length of Tail_

Elongation of the preanal region of the tail resulting in the extension of the cloacal opening beyond the posterior edge of the carapace occurs in males of several kinds of turtles, including _Trionyx_, at least in those from Louisiana, Texas, and Lake Texoma, Oklahoma (Webb, 1956:121). Probably this elongation is characteristic of males of all American softshells. Some females of _spinifer_ and _muticus_ that exceed the maximum size attained by males have the tip of the tail and cloacal opening extending a short distance beyond the posterior edge of the carapace. Some large females of _ferox_ have more elongate tails than those of _spinifer_ and _muticus_.

_Width of Alveolar Surfaces of Jaws_

Stejneger (1944:34-36, pl. 6) commented on a series of large skulls of _ferox_ mostly from Kissimmee, Florida, some of which had conspicuously expanded alveolar surfaces. He suggested that the condition was confined to large males. A scattergram (Fig. 2) based on measurements obtained from 45 skulls of _ferox_ shows widened alveolar surfaces of the upper jaws on some of the larger skulls. Because the maximal size of adult males is unknown and the difference in size between the sexes of _ferox_ is slight, such large skulls might represent either sex. The sex had been recorded for only three of the 45 skulls; none of the three exceeded 82 millimeters in basicranial length or had widened alveolar surfaces. Some of the larger skulls of approximately the same size differ markedly in width of the alveolar surfaces; this difference suggests that both sexes are included and that the sexes may be of approximately the same maximal size. On the other hand, the variation observed in skulls is possibly confined to one sex. To judge from what is known of the maximal sizes of the sexes of _spinifer_ and _muticus_ (see Table 2), skulls of _ferox_ of more than 85 millimeters in basicranial length probably are of females. The largest alcoholic male (dissected) of _ferox_ that I examined had a width of head of approximately 46.5 millimeters; that measurement corresponds to a basicranial length of 70 to 75 millimeters. The specimen of which measurements are depicted by the uppermost symbol in the scattergram (represented by KU 16528) was recorded as a female. Large females of _T. s. asper_ from rivers emptying into the Atlantic Ocean have broadened alveolar surfaces.

_Length of Claw_

Secondary sexual differences in length of claw on the forelimb are pronounced in some kinds of turtles. Cahn (1937:178) stated that the female of _Trionyx muticus_ usually has long claws on the hind feet, while the male has long claws on the forefeet, but I am unable to substantiate his statement. Measurements of length of the third claw on the hind limb taken in 41 males and 45 females of _spinifer_ from Louisiana showed no secondary sexual difference.

Ontogenetic Variation

_Pattern_

In all species and subspecies the juvenal pattern is replaced in females as growth proceeds by a mottled and blotched pattern that is contrasting or of nearly uniform coloration. The blotched pattern (of lichenlike figures) is evident on the carapaces of most females that have plastra so long as 8.0 centimeters. The contrasting juvenal pattern on the dorsal surfaces of the soft parts of the body is correspondingly modified in females, but at a size larger than 8.0 centimeters. Size of ocelli (OD/PL) in _T. s. spinifer_ and _hartwegi_ seems to vary ontogenetically (see section on Geographic Variation).

Some hatchlings have blotched patterns (_T. spinifer asper_, TU 16689.2, plastral length, 3.5 cm.); the largest females examined that did not show any evidence of mottling were two _asper_ having plastrons 7.6 and 8.0 centimeters in length. Variation in color and pattern probably is modified greatly by the environment (Heude _in_ Stejneger, 1907:518, footnote d) and the physiological condition of the individual. Smith, Nixon and Minton (1949:92) reported that a female of _T. s. hartwegi_ developed a striking melanistic pattern in captivity and they concluded that patterns of soft-shelled turtles may be produced not only by conventional chromatophores, but also by other depositions, both intra- and extracellular. TU 16170, taken from brackish water at Delacroix Island, St. Bernard Parish, Louisiana, is the only adult male I have seen that had a blotched pattern (orange-brown in life) on the carapace in addition to the juvenal pattern. One female of _muticus_, KU 48229, having a plastral length 14.5 centimeters, retained a well-defined juvenal pattern, and lacked a mottled and blotched pattern (see Pl. 46).

_Tuberculation_

Males of the subspecies of _spinifer_ develop small, sharp tubercles on the dorsal surface of the carapace when sexually mature. As growth proceeds, the minute prominences along the anterior edge of the carapace on hatchlings of both sexes of _spinifer_ change in shape to conical projections or low, flattened, scarcely-elevated prominences, depending on the subspecies (Fig. 8).

Large females of _spinifer_ and _ferox_ acquire enlarged, flattened knobs in the nuchal region and posteriorly in the center of the carapace.

_Length of Tail_

The preanal region of the tail rapidly elongates in males of all soft-shells when they are sexually mature.

_Width of Alveolar Surfaces of Jaws_

The alveolar surfaces of the jaws are conspicuously broadened in large adults of _ferox_, and females of that population of _T. s. asper_ in the Atlantic Coast drainage.

_Ratios_

Width of head increases at a rate slightly slower than does the length of the plastron (PL/HW, Fig. 3). The change in proportions is most pronounced at a plastral length of 7.5 to 8.0 centimeters. In general, the head is narrowest in _muticus_ and widest in _ferox_. _T. s. asper_ and _emoryi_ seemingly have the widest heads among the subspecies of _spinifer_. Geographically width of head increases from _spinifer_ and _hartwegi_ through _pallidus_ and _guadalupensis_ to _emoryi_. _T. ater_ terminates the cline; 12 specimens, ranging in plastral length from 9.6 to 18.4 centimeters, resemble _ferox_ and _asper_ in having wide heads (average PL/HW of 4.93).

The carapace increases in width more slowly than it increases in length (CL/CW, Fig. 4). The change in proportions is most pronounced when the carapace is 8.0 to 8.5 centimeters in length. Ontogenetically _muticus_ varies least and _ferox_ most; large specimens of _ferox_ have narrower carapaces than _muticus_ of corresponding size. There is also an indication of a geographical gradient that parallels the cline mentioned above for PL/HW. There is a gradual decrease in width of carapace from _pallidus_ through _guadalupensis_ to _emoryi_. Of the subspecies of _spinifer_, _emoryi_ has the narrowest carapace and resembles _ferox_. In _T. ater_ this cline is accentuated and terminates; 12 specimens, ranging in plastral length from 9.6 to 18.4 centimeters, resemble _ferox_ and _emoryi_ in having narrow carapaces (average CL/CW of 1.32).

_Osteological Characters_

Closure of the anterior, paravertebral fontanelles on the bony carapace, and size and number of plastral callosities are subject to ontogenetic variation (see sections entitled "Carapace" and "Plastron").

a. _T. ferox_ (UMMZ 102276, × 1/3)
b. _T. spinifer hartwegi_ (KU 46742, × 3/4)
c. _T. spinifer asper_ (KU 50842, × 1)
d. _T. spinifer pallidus_ (KU 2958, × 1/2)
e. _T. spinifer pallidus_ (KU 2934, × 1/2)
f. _T. spinifer pallidus_ (KU 2947, × 1/2)
g. _T. spinifer guadalupensis_ (TU 10165, × 2/3)
h. _T. spinifer emoryi_ (KU 48218, × 2/3)
i. _T. muticus muticus_ (KU 48236, × 2/3)
]

Geographic Variation

Geographic variation occurs in _Trionyx spinifer_ and _T. muticus_. The variant populations of _spinifer_ are segregated into six subspecies, those of _muticus_ into two. In the subspecies of _spinifer_ there is both group variation and clinal variation.

Group Variation

The six subspecies of _spinifer_ can be separated into two groups on the basis of the juvenal pattern. One group (subspecies _spinifer_, _hartwegi_ and _asper_) has a pattern of dark spots or ocelli of various sizes on the carapace, whereas the other group (subspecies _pallidus_, _guadalupensis_ and _emoryi_) has a pattern of small white dots or tubercles on the carapace. The two groups differ also in the manner in which the mottled and blotched pattern first appears on the carapace of females. Usually, contrasting lichenlike figures initially surround the dark spots or ocelli on the carapace in females of the _spinifer_ group (less evident in _pallidus_), whereas females of the _emoryi_ group usually lack a contrasting pattern early in ontogeny. In general, the two groups differ in the degree of pigmentation. The _spinifer_ group has larger marks and more contrasting patterns on the head and limbs, and more extensive pigmentation on the ventral surface than members of the _emoryi_ group. _T. ater_ is more closely related to those subspecies of the _emoryi_ group but differs in having the ventral surface heavily speckled with black and an over-all blackish, dorsal coloration; the underlying pattern of _ater_ resembles that of _emoryi_.

Clinal Variation

Several characters are arranged in a geographical gradient or cline. Some characters are relatively uniform and represent a terminus in the _spinifer_ group. Some characters change gradually and successively through the subspecies _pallidus_ and _guadalupensis_, and terminate in _emoryi_ and _T. ater_. Some characters of _ater_, in turn, show affinity with _T. muticus_ and _T. ferox_.

_Pattern on Snout_

The pattern (Fig. 5) on the snout usually consists of pale, dark-bordered stripes that form an acute angle in front of the eyes in _spinifer_, _hartwegi_ and _asper_, but the corresponding marks form a dark triangle the base line of which joins the anterior margins of the orbits in _emoryi_ and usually in _guadalupensis_. In _pallidus_, the geographic range of which is between _guadalupensis_ and _hartwegi_, there are different patterns that are in various degrees intermediate between those described immediately above for _hartwegi_ and _guadalupensis_.

_Pattern on Side of Head_

The change in pattern (Fig. 6) and its contrast with the ground color on the side of the head parallels the sequence of changes in pattern on the snout. The pattern on the side of head contrasts with the ground color and consists of dark markings below the eye and on the neck, an indication of a postlabial stripe, and a pale, dark-bordered postocular stripe that may be variously interrupted (_spinifer_ and _hartwegi_; _asper_ usually has uninterrupted postocular and postlabial stripes that unite on the side of the head). The pattern is contrasting but variable in _pallidus_. _T. s. emoryi_ and usually _guadalupensis_ have fewer dark markings, sometimes none, and an interrupted postocular pale stripe that produces a pale blotch just behind the eye.

a. _T. ferox_ (UMMZ 102276, × 1/3)
b. _T. spinifer spinifer_ (UMMZ 54401, × 2/3)
c. _T. spinifer asper_ (KU 50843, × 2/3)
d. _T. spinifer pallidus_ (KU 50830, × 3/4)
e. _T. spinifer guadalupensis_ (SM 659, × 2/3)
f. _T. spinifer emoryi_ (KU 2922, × 3/4)
g. _T. muticus muticus_ (KU 48228, × 2/3)
h. _T. muticus calvatus_ (KU 47117, × 2/3)
]

a. _T. spinifer hartwegi_ (KU 15932, × 3/4)
b. _T. spinifer pallidus_ (KU 40175, × 2/3)
c. _T. spinifer guadalupensis_ (TU 10165, × 3/4)
d. _T. spinifer emoryi_ (KU 3153, × 5/6)
e. _T. muticus muticus_ (KU 48228, × 3/4)
f. _T. ferox_ (UMMZ 102276, × 1/2)
]

a. _T. ferox_ (UMMZ 90010)
b. _T. spinifer hartwegi_ (KU 3346)
c. _T. spinifer pallidus_ (TU 13213)
d. _T. spinifer guadalupensis_ (TU 10160)
e. _T. spinifer emoryi_ (KU 2906)
f. _T. ater_ (KU 46906)
g. _T. muticus muticus_ (KU 48229)
h. _T. muticus muticus_ (KU 48232)
]

_Pattern on Dorsal Surface of Limbs_

A corresponding sequence of change occurs in the size of dark markings on the dorsal surface of the limbs (Fig. 7). The hind limb usually has larger markings than the forelimb. The change is gradual from larger and darker markings (contrasting pattern) in _hartwegi_, _spinifer_ and _asper_ to smaller and paler markings (non-contrasting pattern) in _emoryi_.

_Tuberculation_

There is also a cline in tuberculation (Fig. 8) that parallels geographically the sequence of changes in patterns mentioned immediately above. The size of the tubercles along the anterior edge of the carapace changes in both sexes from those that are enlarged and equilateral or conical in shape in _spinifer_, _hartwegi_, _asper_ and _pallidus_ to those that are scarcely elevated in _guadalupensis_, _emoryi_ and _T. ater_. Indeed, in the three kinds mentioned last, the tubercles are absent in some specimens. There seems to be a corresponding reduction in the size and number of small, sharp-tipped tubercles that cover the carapace in adult males; the carapace of _T. ater_ is mostly smooth and has only a few small, whitish tubercles.

_Ratios_

The clinal tendencies in PL/HW (Fig. 3) and CL/CW (Fig. 4) that parallel those mentioned above for pattern and tuberculation have already been mentioned under the section "Ontogenetic Variation."

The ratio of CL/PCW (Fig. 9) was used in an effort to show further differences in the shape of the carapace, especially the plane on the carapace where the greatest width occurs. Figure 9 shows the greatest width to be approximately midway between the anterior and posterior ends in the subspecies _spinifer_, _hartwegi_ and _asper_, and in the species _ferox_ and _muticus_ (CL/PCW of 2.00). The greatest width of carapace is more posterior and at approximately the same plane in _pallidus_ and _guadalupensis_, and farther posterior in _emoryi_. Calculated ratios for 12 specimens of _T. ater_ average 2.15, a value that suggests closer affinity with _pallidus_, _guadalupensis_ and _emoryi_ than to the other species and subspecies.

Comparison of the relative lengths of snout (HW/SL, Fig. 10) in different populations of _T. spinifer_ shows a character gradient. To facilitate a comparison utilizing large samples, the subspecies _spinifer_ was combined with _hartwegi_, and _pallidus_ with _guadalupensis_. The snout is longer in the subspecies _spinifer_ and _hartwegi_ than in _emoryi_; the length of the snout of _emoryi_ resembles that of _T. ferox_. The snout is proportionately the longest in _T. muticus_. The average ratio of HW/SL for 12 individuals of _T. ater_ is 1.37, and is nearer that of _pallidus_, _guadalupensis_, _emoryi_ and _ferox_ than that of _muticus_ or the other subspecies of _T. spinifer_.

Size of the ocelli increases from west to east in populations of _T. spinifer_ in the upper Mississippi River and Great Lakes drainages.

The ratio of OD/PL (Fig. 11) varies considerably but gradually increases from Kansas northeastward to Michigan. The minimal diameter of any ocellus recorded was one millimeter; solid dots on the carapace (_hartwegi_) were also recorded as one millimeter. Larger ratios are usually derived from measurements of larger individuals. Seemingly, there should be a clinal tendency in ontogenetic variation paralleling the size of ocelli and dependent on it; ontogenetic variation should be least in western populations in which the size of ocelli does not change appreciably with increasing size, and should be greatest in eastern populations in which the ocelli on adult males are larger than those on the carapace of young turtles. It is difficult to demonstrate ontogenetic variation because specimens of corresponding size from the same general area may have ocelli of different sizes. The gradient in size of ocelli is also indicated by specimens from other states. I have the subjective impression that there is least variation in specimens from Michigan (Great Lakes-St. Lawrence River drainage), but this is not clearly shown by Figure 11.

Character Analysis

_Snout_

Comments

North American Recent Soft-Shelled Turtles (Family Trionychidae)Chapter I: Part 1

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