The Project Gutenberg eBook of North American Recent Soft-Shelled Turtles (Family Trionychidae)

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

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1962

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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.

Month	Gill-net days	Number of turtles	Gill-net days 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 [438] 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 [439] 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 ʌ-shaped epiplastron, [440] 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).

Fig. 1. Geographic distribution of the family Trionychidae.

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.