Chapter VIII: Part 8

The data concerning _ferox_ (Table 8) suggest that the maximum size of eggs is 31 to 32 millimeters, whereas oviducal eggs are slightly smaller, about 25 to 27 millimeters. Eggs of _spinifer_ from northern latitudes (most from approximately 40°, Table 8) also vary in size, oviducal eggs being as small as 22 millimeters in diameter and the maximal size about 29 millimeters. Average extreme measurements (in mm.) of oviducal eggs (number of eggs in parentheses) from females taken in latitudes of 33 degrees or less are: 25 × 29 (11), 29 × 30 (11), 28 × 30 (13), 28 × 30 (10), 29 × 30 (5), 29 × 29 (8), 25 × 26 (17), 29 × 30 (5), and 28 × 29 (8). The average size of these eggs is slightly larger than the oviducal eggs of which measurements are given in Table 8, and suggest larger eggs from more southern latitudes. Eggs of _muticus_ are smaller than those of _spinifer_ (Cahn, 1937:183) or _ferox_; the average size of eggs from nests found in Iowa and Illinois is 22 to 23 millimeters (Table 8). Nine oviducal eggs from a female obtained in Lake Texoma, Oklahoma, averaged 22 × 23 millimeters. The largest eggs of _muticus_ are from the southernmost locality; eight eggs from a nest found along the Escambia River, Florida, averaged 26 × 27 millimeters.

In general, the data suggest that at each laying slightly smaller eggs but larger numbers are laid by females in northern latitudes, whereas larger but fewer eggs are laid by females from farther south.

_Incubation and Hatching_

Length of the incubation period seems to depend upon conditions of heat and moisture, and, in general, to be geared to the prevailing climatic conditions. Goff and Goff (1935:156) artificially incubated some eggs of _ferox_ at temperatures varying from 82.3 to 89.2° F., and found that the incubation period was 64 days. Muller (1921:184) wrote that the period of incubation of eggs of _muticus_ (natural nests at temperatures about 90°., _op. cit._:182, and artificial nests) in Iowa is from 70 to 75 days. Breckenridge (1944:187) stated that _spinifer_ makes nests in Minnesota from June 14 to July 6, and cited reports that indicate hatching in September. Hedrick and Holmes (1956:126) discovered a nest of eggs in Minnesota on September 5; the eggs were artificially incubated and some hatched on October 29. Eigenmann (1896:263) found eggs as late as September in northern Indiana that "contained young which would have been ready to hatch about a month later." Cahn (1937:193) wrote that _spinifer_ in Illinois lays in June or early July and that "young-of-the-year are taken in late August and September." Some recently deposited eggs of _muticus_ (as indicated by fresh turtle tracks, Pl. 50, Fig. 2) that I obtained on June 1 were artificially incubated and hatched on August 4, indicating an approximate incubation period of 65 days. Dr. Paul K. Anderson in the course of field work on the Pearl River, Louisiana (1958:211), found that eggs collected on June 13 from a nest excavated three to five days before, hatched on August 15, indicating an incubation period of approximately 67 days. Eggs collected on May 17 to 25 (three clutches) hatched on August 4 to 6, indicating an incubation period of approximately 77 days. In any latitude the incubation period probably is at least 60 days. Eigenmann (_loc. cit._), however, mentioned empty nests that were found in July; this indicates early hatching or more probably the action of predators.

In northern latitudes eggs or young turtles may over-winter in the nest if deposition is late in the season. In northern Indiana Evermann and Clark (1920:595) found a nest on November 16 that contained "well-formed young" and believed that the turtles would have wintered in the nest. Conant (1951:160) was of the opinion that most eggs probably hatch in early fall, but that some do not hatch until spring.

The hatching of eggs of _muticus_ has been described by Muller (1921:183). According to him, the forelimbs first emerge through the shell and enlarge the opening. There is an "egg tooth below the flexible proboscis" but "it does not seem to be used in escape from the eggs, and is dropped a week after hatching." Hatchlings burrow almost straight upward through the sand leaving the egg shell below the surface and a hole in the sand about an inch in diameter. Muller found that young turtles emerge from the nests in the night or early morning and always go downhill probably influenced in their movements by the open sky and sloping beach. Anderson (1958:212-15) found that hatchlings of _muticus_ leave nests within the first three hours after sunset and travel a direct route to the water. He discovered that hatchlings are active on the surface of the sand at night and generally show a positive reaction to light (moonlight, flashlight), whereas, in daytime, there is a negative reaction to bright sunlight (causing the turtles to bury themselves in sand). Anderson believed that the positive response to light at night is not correlated with the water-approach behavior of hatchlings, but that movements to water are possibly influenced by a negative reaction to dark masses of environment (such as shadows formed by landward forests).

_Age and Growth_

Goff and Goff (1935:156) found that hatchlings of _ferox_ average 8.82 grams (extremes, 8.50 to 9.25); one of these, UMMZ 76755, is illustrated in Plate 31. Muller (1921:184) recorded measurements of five hatchlings of _muticus_; the average measurements (in cm., extremes in parentheses) were: length of carapace, 3.54 (3.43 to 3.67); width of carapace, 3.20 (3.10 to 3.25); length of plastron, 2.54 (2.47 to 2.60). I recorded measurements of 32 hatchlings (three clutches combined) of _muticus_ on August 16; the turtles hatched on August 4 to 6 from eggs collected along the Pearl River, Louisiana. The average measurements (in mm., extremes in parentheses) of the 32 turtles were: length of carapace, 41.3 (34.0 to 45.0); width of carapace, 38.6 (31.0 to 40.0); length of plastron, 30.1 (25.0 to 32.0). These turtles have circular umbilical scars averaging approximately two millimeters in diameter. The smallest hatchling that I have seen measures 21.0 millimeters in plastral length (_T. m. muticus_, INHS 3458). There are no data to indicate a difference in size of hatchlings among the American species of soft-shelled turtles. The average plastral length of most hatchlings probably is 28.0 to 30.0 millimeters.

Owing to the lack of a horny epidermal covering of the carapace and plastron, soft-shelled turtles are not so well suited to studies of age and growth as are the "hard-shelled" species, which have visible impressions of growth annuli on the epidermal scutes. Mattox (1936:255) found annular rings in the long bones of specimens of _Chrysemys_ and suggested that it is tenable to correlate the number of rings with the age of the turtle.

Mitsukuri (1905:265) reported that in hatchlings of _Trionyx sinensis_ the length of the carapace averages 2.7 centimeters (hatchlings of _sinensis_ seem to average smaller than any American species), and that the average length of carapace (cm.) at the end of the first year is 4.5, second year 10.5, third year 12.5, fourth year 16.0, and end of fifth year 17.5; he stated also that females of _sinensis_ are sexually mature in their sixth year. Breckenridge (1955:7-9) computed a growth curve based on 11 recaptures of females of _spinifer_ in Minnesota; his data on rate of growth for the first five years do not differ appreciably from those of Mitsukuri. As most females of _spinifer_ are sexually mature when the carapace is about 11 inches long, the age at sexual maturity is approximately 12 years according to Breckenridge (_op. cit._:8, Fig. 4). The discrepancy in age of females at the size of attainment of sexual maturity (Mitsukuri--six years; Breckenridge--12 years) is, in part, rectified by the fact that _Trionyx sinensis_ probably is a smaller species. Also, Breckenridge's computation of the growth curve is based on continuously decreasing increments of growth and seemingly eliminates consideration of the probable marked decrease in rate of growth that occurs when sexual maturity is attained--a phenomenon noted in other species of turtles. I think that increments of growth of immature turtles are, on the average, larger than those of sexually mature turtles. Judging from these criteria, the age of a female of _spinifer_ at sexual maturity is less than 12 years, and turtles having carapaces 17 to 18 inches in length (maximal size for _spinifer_) would be older than 53 years (_op. cit._:9). Occasional individuals, however, may greatly exceed the usual growth rate in which event large adults may be younger than 50 years.

Females of _muticus_ are sexually mature when the plastron is 14.0 to 16.0 centimeters long, which corresponds to a carapace 19.6 to 22.4 centimeters (about 7-3/4 to 8-3/4 inches) long (average CL/PL approximately 1.4, see Fig. 13). The smaller adult females probably mature sexually in their sixth year, but most probably do so when seven years old. Accordingly, some _T. spinifer emoryi_, which are sexually mature at a plastral length of 16.0 centimeters, are also sexually mature in their seventh year, whereas most _spinifer_ (sexually mature at a plastral length of 18.0 to 20.0 cm., corresponding to a length of carapace of 25.2 to 28.0 cm. or about 10 to 11 inches) probably become sexually mature in their ninth year, and some when eight years old. Most males of _spinifer_ are sexually mature when the plastron is 9.0 to 10.0 centimeters long (length of carapace 12.6 to 14.0 cm. or 5 to 5-1/2 inches), whereas males of _muticus_ and some _T. spinifer emoryi_ are sexually mature at a plastral length of 8.0 to 9.0 centimeters (length of carapace 11.2 to 12.6 cm. or 4-1/2 to 5 inches). The smaller adult males are probably sexually mature in their fourth growing season. Breckenridge (_op. cit._:7, Tab. II) commented on the abundance of females between five and 12 inches in length, and males that ranged in length from five to seven inches. The abundance of turtles in these size ranges is probably due, in part, to a slowing of the rate of growth indicating the approach of sexual maturity; the abundance of the smallest males is especially in accord with the size at sexual maturity of males (about five inches).

The largest acceptable record of size of _spinifer_ is 18 inches in length of carapace (Breckenridge, 1957:232). Stockwell (1878:402), however, wrote that females of _spinifer_ attain "an extreme length of from twenty-four to twenty-eight, and, in rare instances, thirty inches, with an average length of carapace of fifteen to eighteen," and True (1893:152) mentioned lengths of two feet or even more. Turtles 17 to 18 inches long are doubtless rare and probably about 60 years old. A specimen of _ferox_ lived the longest time in captivity--25 years (Pope, 1949:304). Individuals of _ferox_ probably exceed the maximum recorded length of carapace of 18-1/2 inches (Agassiz, 1857:401). The head of a _ferox_ having a width of 3-1/2 inches (Wright and Funkhouser, 1915:120) corresponds to a length of carapace of approximately 22-1/2 inches (PL/HW == 4.9; CL/PL == 1.3). De Sola and Abrams (1933:12) wrote that _ferox_ in the Okefinokee Swamp, Georgia, attains a length of two feet. The largest female of _muticus_ of which I have record is 21.5 centimeters in plastral length (KU 2308), a measurement corresponding to a carapace about 13 inches long.

Mortality

Man, in one sense or another, is a great enemy of soft-shelled turtles. Those caught by fishermen are destroyed because of the erroneous belief that they are harmful to fish populations. Some are drowned in hoop-nets or gill nets used by commercial fishermen. Many softshells are used by man for food. Herald (1949:118-19) reported the results of spraying an area with DDT and mentioned a 10-inch individual of _ferox_ that was eating a dead bluegill, and which "probably died as a result of ingesting contaminated food."

Predation on eggs probably accounts for most mortality. Hamilton (1947:209) reported tracks of spotted skunks, raccoons and foxes seen about destroyed nests, and Cahn (1937:183) incriminated skunks and raccoons. Goldsmith (1945:449) reported a raccoon that unearthed seven nests in one night. Little and Keller (1937:221) wrote of egg shells found in the sand (probably not as a result of hatching), and Muller (1921:182) reported egg shells around dug-up nests, suggesting such predators as "ground moles," raccoons and crows. Chesser (_in_ Harper, 1926:416) said that in the Okefinokee Swamp the jackdaw (fish crow), raccoon, bear and domestic dogs will eat the eggs. Wright and Funkhouser (1915:122) recorded a young _ferox_ in the stomach of a water moccasin (_Agkistrodon piscivorus_), and suggested that young soft-shells probably are food of larger snakes. Kellogg (1929:26) wrote that stomachs of two alligators each contained one soft-shelled turtle. Newman (1906:136) found that young captives were eaten by individuals of _Chrysemys_ and _Sternothaerus_, and I found that they were eaten by _Kinosternon_. Mitsukuri (1905:261-62) stated that first- and second-year individuals of _T. sinensis_ are eaten by the adults.

Breckenridge (1960) wrote that a clutch of eggs probably failed to develop because of an "... unusually cool season." Evermann and Clark (1920:595) stated that "many young appear to perish during the first winter." They (_op. cit._:594) found two eggs submerged in two feet of water and it is supposed that they never hatched. Dundee (1950:139) reported remains of soft-shelled turtles left on the mud of a dried swamp.

Parasites

Muller (1921:182) found maggots in a few eggs of a clutch, but thought that only the infertile and decomposing eggs were infested. I removed a hard, spherical cyst from the hind leg of a preserved softshell (TU). A captive hatchling (TU 17304) died as the result of a continuously enlarging and deepening hole on the top of its head; I could not discern a visible parasite with the naked eye. I found 25 leeches (_Placobdella parasitica_, largest about 13 mm.; identified by Dr. Kenneth B. Armitage, Department of Zoology, University of Kansas) in association with 11 _T. m. muticus_ (number per turtle not known) that were collected from the Kansas River at Lawrence, Douglas County, Kansas. Evermann and Clark (1920:596) reported a few nematodes in the stomachs of some _spinifer_, and three nematodes are listed by Harwood (1932:46, 60, 62, 66) in the same species. Hughes, Higginbotham and Clary (1941) have listed the known reptilian hosts of parasitic trematodes, and Hughes, Baker and Dawson (1941) have done the same for tapeworms. The species of parasites and their trionychid hosts are listed below.

TREMATODA
_Trionyx ferox_: _Neopolystoma orbiculare_ _Vasotrema amydae_
_Neopolystoma rugosa_ _Vasotrema attenuatum_
_Polystomoides coronatus_ _Vasotrema robustum_
_Teloporia aspidonectes_

_Trionyx muticus_: _Crepidostomum cooperi_ _Opisthorchis ovalis_

_Trionyx spinifer_: _Hapalorhynchus evaginatus_ _Vasotrema amydae_
_Opisthorchis ovalis_ _Vasotrema attenuatum_
_Polystomoides coronatus_ _Vasotrema longitestis_
_Teloporia aspidonectes_ _Vasotrema robustum_

CESTODA
_Trionyx ferox_: _Proteocephalus trionychinus_

_Trionyx spinifer_: _Proteocephalus testudo_

NEMATODA
_Trionyx spinifer_: _Camallanus trispinosus_ _Spiroxys amydae_
_Falcaustra chelydrae_

Economic Importance

Several authors have mentioned softshells as a food item much sought after by man. The commercial value of these turtles has been summarized by Clark and Southall (1920:15-16). Softshells are consumed in quantity only in small towns near the place of capture. They are found only occasionally in the markets of large cities because the turtles are little known and the palatability of their flesh is unappreciated. Also, they do not stand shipment so well as other turtles, and they are "not so meaty as the snapper; so there is more waste" (Clark and Southall, _loc. cit._). Little and Keller (1937:221) reported living individuals for sale at the market in Ciudad Juarez, Chihuahua; however my inquiry at markets in Juarez in the summer of 1959 disclosed no evidence of recent sale of soft-shelled turtles. In the southeastern United States the demand is perhaps greater than in other regions. I have noted softshells in the market at New Orleans, and Oliver (1955:19) has mentioned the sale of "some 146,600 pounds" in one recent year in Florida. Over most of their range, however, there probably is no general demand for softshells and no special efforts are made to capture them. Softshells have been raised successfully on "turtle farms" in Japan (Mitsukuri, 1905). True (1893:152) wrote that "The eggs also are considered very excellent."

Softshells generally are condemned by fishermen because of the mistaken belief that they are detrimental to fish populations. Food of softshells is principally crawfish and insects. Fish comprise a small proportion of the diet (frequency 1.9% in Michigan, Lagler, 1943: Tab. 9). Most of the fishes eaten seem to be small minnows. Probably fish would comprise a larger percentage of the diet if they could be caught. I doubt that a softshell can pursue and capture a healthy fish in natural waters. Recently dead fish are eaten and perhaps fish eggs, and senile and decrepit fishes. There is no evidence that soft-shelled turtles are active predators on any kind of fish. Of course in congested areas such as ponds of fish hatcheries, it is desirable to eliminate the turtles. The known food habits of soft-shelled turtles suggest that they compete with game fishes for food, but there is no information on the intensity of competition (Lagler, _op. cit._:305).

The combined statements of many authors in their general accounts of food habits (for instance, Babcock, 1919:425) have tended to create the erroneous belief that soft-shelled turtles harm waterfowl. To my knowledge the only basis for this belief is the statement of Wright and Funkhouser (1915:123) that according to the natives of the Okefinokee Swamp, the larger turtles "devour also such waterfowl as are unfortunate enough to be taken unaware by these reptiles." Perhaps an occasional waterfowl is eaten, but the present information on kinds of food eaten certainly does not warrant the destruction of soft-shelled turtles. There may be some mortality in congested areas such as game refuges where young birds crowd the surface of the water.

The kind of bait successfully used in trapping softshell turtles suggests that they are of some value as scavengers.

EVOLUTIONARY HISTORY

Before attempting to reconstruct the history of soft-shelled turtles in North America, it will be helpful to summarize the salient facts concerning the distribution and relationships of the living forms, and to comment on fossils.

Distribution

The geographic range of the family Trionychidae in North America is principally in the eastern two-thirds of the continent and contributes to the well-known floral and faunal resemblance of eastern North America to that of eastern Asia (Schmidt, 1946:149) because _Trionyx ferox_ (see Fig. 18) resembles the species of the genus in Asia more closely than it does any North American species. The Recent distribution in America does not include the Neotropical region, whereas the geographical range in the Old World extends south of the equator (Fig. 1; Dunn, 1931:109, fig. 2; Gadow, 1909:333, fig. 72; Hay, 1908:35, fig. 16).

American softshells occur in all river systems in the United States and the two adjacent river systems on the east coast of México that drain into the Gulf of México. Softshells inhabit streams of the Great Plains and occur westward to the foothills of the Rocky Mountains in the western tributaries of the Mississippi River. Only _T. s. spinifer_ occurs in the southern part of the Great Lakes-St. Lawrence drainage. Softshells are absent from the Atlantic Coast drainage except the Hudson River and those rivers at least south of (and including) the Pee Dee River in South Carolina.

_T. s. emoryi_ is not known to be indigenous west of the Río Grande drainage, and has probably been introduced across the Continental Divide via the Gila River in western New Mexico into the Colorado River drainage of Arizona (Miller, 1946:46); the species undoubtedly occurs in México on the Sonoran side of the Colorado River opposite Baja California (Bogert and Oliver, 1945:417).

In the summer of 1959, I trapped turtles and with a specimen in hand inquired about softshells occurring in the inland drainages of northern México. From two collecting stations on the Río Nazas in Durango, only specimens of _Pseudemys_ and _Kinosternon_ were obtained; local inhabitants had neither seen nor heard of softshells. Flooded conditions in August of 1959 permitted trapping in only one of the inland drainages of northwestern Chihuahua, the Río Santa María; only specimens of _Kinosternon_ were obtained. Local residents near that river as well as those living near the Río Casa Grandes and Río del Carmen had not seen or heard of softshells. A person that I judge to be a competent observer reported seeing a softshell in June of 1958 in the Río Alamos (Arroyo Cuchujáqui) near Alamos, Sonora, in the Río del Fuerte drainage on the west coast of México. I was a member of a field party from the University of Kansas that visited that locality in late January of 1959; only specimens of _Pseudemys_ and _Kinosternon_ were collected. Possibly isolated populations occur in streams of the Pacific Coast drainage of northern México. If so, they may have entered Pacific Coast drainages by stream capture across the Continental Divide. Several species of fish that are characteristic of the Río Grande traversed the Sierra Madre Occidental at some former time (presumably via the Río Conchos and Río Papigochic) and occur in the Yáqui River drainage (Meek, 1904:xxxviii, xlvii; Miller, 1959:214-15, 217). Because of the probability that the Río Nazas at some former time flowed north into the Río Grande (Meek, _op. cit._:xxxiv), it is notable that softshells are absent in the Río Nazas drainage; the Big Bend turtle, _Pseudemys scripta gaigeae_, occurs in both drainages.

Relationships

Characters of _Trionyx ferox_ suggesting a closer resemblance to some Old World members of the family than to the other three American species are: large size; marked difference between juvenal and adult patterns on the carapace; the marginal ridge; and the longitudinal ridgelike prominences on the carapace, especially in juveniles. Other characters of _ferox_ suggesting a corresponding, but less marked resemblance to Old World species of _Trionyx_ are: the large size of the eighth pair of pleurals; the absence of callosities on the epiplastron and preplastra; frequent fusion of the hyoplastra and hypoplastra (more than in _spinifer_ or _muticus_); and tolerance of marine waters (more than _muticus_ or _spinifer_). Some fossils also suggest alliance with _ferox_ and some Old World members of the genus in their large size, large eighth pair of pleurals, and occurrence in marine deposits; several Old World species have been reported at sea (_Pelochelys_, _T. triunguis_, _T. sinensis_). _T. ferox_ is monotypic and has the most southeasterly displaced, geographic range.

Because _ferox_ resembles softshells from the Old World more closely than it does any American species, _ferox_ is assumed to be more closely related to Old World softshells than to any American species, and, because of resemblance to some fossils, _ferox_ is assumed to resemble most closely the primitive, ancestral stock of softshells that occupied North America. _T. spinifer_, _T. muticus_ and _T. ater_, which resemble each other more closely than any of them resembles _T. ferox_ or any Old World species, are considered autochthonous in North America. _T. spinifer_ and _T. muticus_ are distinct, sympatric species. Burt (1935:321) suggested that the two species "may be variants of the same species." _T. ater_ is weakly differentiated from _T. spinifer emoryi_. The species, _ferox_, _spinifer_ and _muticus_ are well-differentiated and were considered by Agassiz (1857), Gray (1869) and Baur (1893) as belonging to three different genera.

In the widely distributed _T. spinifer_, the subspecies _spinifer_, _hartwegi_ and _asper_ closely resemble one another; _asper_ seems most distinct, whereas _spinifer_ and _hartwegi_ are terminal populations of an east-west cline in one character. The subspecies _pallidus_, _guadalupensis_ and _emoryi_ resemble one another more closely than any resembles any of the subspecies mentioned immediately above; _T. s. pallidus_, however, is annectent. _T. s. pallidus_ and _guadalupensis_ represent terminal populations of clines in several characters, some of which occur in _emoryi_, but that subspecies is more distinct from _pallidus_ and _guadalupensis_ than those subspecies are from each other. _T. s. emoryi_ is the most variable subspecies. _T. ater_, known only from a restricted area in central Coahuila, is most closely related to _T. s. emoryi_, and possesses some characters judged to represent the attenuation of the geographic cline in _pallidus_, _guadalupensis_ and _emoryi_ mentioned above. Some characters of _ater_ show alliance to the species _muticus_. Of _T. muticus_, whose geographic range is removed from that of _ater_, there are two subspecies. Four subspecies of _spinifer_ (_spinifer_, _hartwegi_, _pallidus_ and _asper_) intergrade in the Mississippi River drainage of Louisiana; few specimens, however, are typical of _asper_. The subspecies of _muticus_ do not show definite evidence of intergradation. To facilitate quick reference, the occurrence of some characters that are shared by, or are approximated in, two or more forms are listed in Table 10. In addition to external characters, some ratios emphasize the clinal relationship between _T. s. pallidus_, _guadalupensis_, and _emoryi_ mentioned above. Of especial interest is the frequent resemblance of those subspecies and _T. ater_ to _T. ferox_ (dorsal pattern on limbs of adults, reduction in anterior tuberculation, wide head, narrow carapace, and short snout), and the less marked resemblance of _T. muticus_ to _T. ferox_; not shown in Table 10 is the resemblance of _ferox_ to _T. muticus calvatus_ in having thick, black-bordered postocular stripes. Some populations of _T. s. emoryi_ resemble _T. muticus_ in the corresponding size at sexual maturity and in having well-developed plastral callosities. It is notable that the occurrence of _ater_, and to a lesser extent that of _T. s. emoryi_, which resembles _ferox_ (and _muticus_), is in the southwestern United States and northern México.

TABLE 10. Frequency of Selected Characters Among Species and
Subspecies of Trionyx in North America. Characters of muticus
Refer to the Typical Subspecies; Horizontal Dashes Connecting
X's Indicate that Computations for Those Subspecies Were
Combined; Vertical Dashes Indicate that the Subspecies Is
Intermediate Between the Adjacent Subspecies.

Column headings:

A: _ferox_
B: _spinifer_
C: _hartwegi_
D: _asper_
E: _pallidus_
F: _guadalupensis_
G: _emoryi_
H: _ater_
I: _muticus_

=====================================+===================================
| Species and subspecies
Characters +---+---+---+---+---+---+---+---+---
| A | B | C | D | E | F | G | H | I
-------------------------------------+---+---+---+---+---+---+---+---+---
Juvenal pattern: | | | | | | | | |
black spots, ocelli | | X | X | X | | | | |
| | | | | | | | |
white dots | | | | | X | X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---
Pattern on snout: | | | | | | | | |
acute angle (reduced in _muticus_) | X | X | X | X | X | | | | X
| | | | | ¦ | | | |
triangular | | | | | X | X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---
Pattern on side of head: | | | | | | | | |
contrasting marks | X | X | X | X | X | X | | |
| | | | | | ¦ | | |
non-contrasting marks (distinct | | | | | | ¦ | | |
stripe in _muticus_) | | | | | | X | X | X | X
-------------------------------------+---+---+---+---+---+---+---+---+---
Pattern on limbs of adults: | | | | | | | | |
contrasting | | X | X | X | X | X | | |
| | | | | | ¦ | | |
reduced or absent | X | | | | | X | X | X | X
-------------------------------------+---+---+---+---+---+---+---+---+---
Tuberculation (anterior edge of | | | | | | | | |
carapace): | | | | | | | | |
conical, equilateral | | X | X | X | X | | | |
| | | | | | | | |
reduced or absent | X | | | | | X | X | X | X
-------------------------------------+---+---+---+---+---+---+---+---+---
Head (PL/HW, fig. 3): | | | | | | | | |
wide | X | | | X | | X | X | X |
| | | | | | | | |
narrow | | X | X | | X | | | | X
-------------------------------------+---+---+---+---+---+---+---+---+---
Carapace (CL/CW, fig. 4): | | | | | | | | |
wide | | X | X | X | X | X | | | X
| | | | | | ¦ | | |
narrow | X | | | | | X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---
Level of Carapace Width (CL/PCW, | | | | | | | | |
fig. 5): | | | | | | | | |
middle of carapace | X | X | X | X | | | | | X
| | | | | | | | |
farther posteriorly | | | | | X | X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---
Snout (HW/SL, fig. 6): | | | | | | | | |
long | | X---X | X | X---X | | | X
| | | | | ¦ ¦ | | |
short | X | | | | X---X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---

Fossils

The known occurrence of fossil trionychids throughout the world indicates a former distribution more widespread than the family has today; the principal difference in the former and present distributions is the lack of living softshells in Europe.

I have not studied in detail the many fossil remains but such examination as I have made of them suggests that many of the characters used as a basis for distinguishing fossil forms in North America are subject to individual variation or are of no diagnostic value in the living species (Hummel, 1929:769). Knowledge of the variation in the living species of the Old World would aid in adequately appraising the North American fossils. Some osteological characters of the three living American species (excluding _ater_) together with data on variation within a given species are mentioned below. Some differences in skulls of the three species already were mentioned in the section "Osteological Characters." Because most fossil remains are those of the carapace and plastron, attention is here given to those structures.

_Widened alveolar surfaces of jaws._--An ontogenetic variation affecting large skulls of _T. ferox_ and some individuals of _T. spinifer asper_; presumably confined to females. Of especial interest is its presence in some populations of _asper_ that are not otherwise distinguishable (external characters) from the rest of the individuals comprising that subspecies.

_Sculpturing._--No differences in pattern (generally of anastomosing ridges) on carapace or plastron; fineness or coarseness seemingly correlated with size; frequency and kind (knoblike or ridgelike) of bony prominences on carapace variable; bony prominences confined to species _spinifer_ and _ferox_, occurring principally on large females.

_Fontanelles of carapace._--Closure more or less correlated with increasing size, although much variation noted between individuals of same size; small individuals have fontanelles confluent (medially), thus separating nuchal from contact with first neural and first pair of pleurals.

_Number and arrangement of neurals and pleurals._--Neurals number six to nine, usually seven or eight; pleurals number seven or eight pairs, and may or may not be in contact with each other posteriorly; eighth pair of pleurals when present reduced, never contacting seventh neural; arrangement posteriorly variable (see Fig. 16 and Tab. 5).

_Plastral callosities._--Increase in size with advancing age causing corresponding reduction in size of plastral vacuity; relatively best developed in _muticus_ (all elements touching medially on KU 41380 leaving no plastral vacuity); probably no callosities on preplastra or epiplastron of _ferox_; callosity on epiplastron of _spinifer_ not covering entire surface (as it may in _muticus_).

_Epiplastron._--Obtusely-angled (greater than 90 degrees) in _muticus_; acutely-angled (90 degrees or less) in _ferox_ and _spinifer_.

_Hyo-hypoplastral suture._--Usually present, but occasionally absent, in all species.

The fossil turtles of North America have been treated monographically by Hay (1908), who apportioned fossil trionychid remains into eight genera (three living) of two families. Recently, Romer (1956:514) relegated all trionychid fossils to the genus _Trionyx_. Characters, as gleaned from Hay's synopsis (_op. cit._:465-548, Pls. 85-113), that seem especially worthy of taxonomic consideration are: (1) The presence of a preneural, which is not known to occur in the living American species (seemingly the preneural is fused with the first neural and represents the elongate first neural in living species); (2) The large eighth pair of pleurals, especially when they contact the seventh neural; (3) The thickness of the costal plates, a condition probably correlated with the size of some fossils, which are larger than any living species (for example, Hay, _op. cit._:518, mentioned the greatest dimension of a nuchal bone as approximately 300 mm.).

The approximate extent of the known horizontal distribution of fossils is indicated in Figure 24. A comparison of known localities of fossils and the distribution of living softshells (introduced population of _T. s. emoryi_ in Colorado River drainage omitted) shows that the distribution was more widespread in former times. Localities of fossils are centered on the Atlantic Coast from New Jersey to North Carolina and in the Rocky Mountain-Great Plains region from Alberta and Saskatchewan to northwestern New Mexico; the oldest fossils, which occur in each region, are found in Upper Cretaceous deposits. Many fossils occur in marine and brackish water deposits. Most localities depicted on the map are mentioned by Hay (1908:36-37, 465-548). Other localities included on the map are in southern Alberta (Russell, 1929:164; 1930:27; Sternberg, 1926:104), southern Saskatchewan (Russell, 1934:109), northern South Dakota (Hay, 1910:324), central Utah (Gilmore, 1946), western Colorado (Schmidt, 1945), southwestern Kansas (Galbreath, 1948:284), southeastern Texas (Hay _in_ Stejneger, 1944:65), southern California (Brattstrom, 1958:5), and northeastern Coahuila, México (Mullerried, 1943:623). Hay's record of the living _Platypeltis_ (= _Trionyx_) _ferox_ and other remains from the Peace Creek formation in Hillsborough County, Florida (_op. cit._:548), presumably is the same record mentioned by Pope (1949:305).

Ameghino (_in_ Hay, _op. cit._:35) recorded specimens of a trionychid from the Cretaceous of Patagonia, a record that, at present, cannot be accepted (Simpson, 1943:423). Mullerried (_loc. cit._) also mentioned some trionychid remains that were housed in Tuxtla Gutierrez, Chiapas, México, (material now lost), but their geographical provenance was unknown. The former extent of range southward is not known; it is improbable that trionychids occurred in South America (Simpson, 1943:423).

Phylogeny

The occurrence of _T. ferox_ in Florida and the suggestion of _ferox_-like characters in turtles from southwestern Texas and northern Mexico presents a distributional pattern that resembles the disjunct ranges of many other pairs of closely related taxa. The clear-water ponds in central Coahuila, which are inhabited by _ater_, correspond to aquatic habitats supporting _ferox_ in Florida. The splitting of the geographic range into eastern and western parts possibly resulted from a southward shift of colder climates in glacial stages of the Pleistocene, or from the development of an intervening arid region in the late Miocene and Pliocene (see discussions in Martin and Harrell, 1957, and Blair, 1959). An initial separation of range by an arid environment in the Pliocene may have been terminated by the colder climates in the Pleistocene.

The degree of morphological difference between _ferox_ and the forms in southwestern Texas and northern México, suggests that the time of separation antedated the Pleistocene.

Trionychid turtles may have traversed the Bering land bridge between Asia and North America in late Mesozoic times for they occur as fossils on the Atlantic Coast and in the Rocky Mountain-Great Plains region in Upper Cretaceous deposits. Shallow, inland seas may have afforded no barrier to the dispersal of softshells which presumably were tolerant of saline waters. The orogeny and volcanic action with subsequent erosion and sedimentation of the Rocky Mountain system, which was later accompanied by drier climates, tended to obliterate suitable habitats in the western United States; softshells persisted at least until the Upper Eocene on the west coast (Brattstrom, 1958:5). The factors responsible for the disappearance of softshells on the Atlantic Coast probably were related to the glacial advances in the Pleistocene; the most recent fossils known occur in Miocene deposits.

The relationships of the living species and subspecies were probably correlated with geologic change in aquatic environments and drainage patterns. These changes probably included stream capture, flooding, drought, uplifting and planation. A hypothetical, evolutionary history is presented in the phylogenetic diagram where letter symbols represent species and subspecies, and grouped symbols (referred to in subsequent paragraphs) represent ancestral stocks.

Pliocene Pleistocene Recent
==========================================================================

+--F-----------------------------------F (_ferox_)
|
| +---------Mm (_muticus muticus_)
| +--M------------------+
| | +---------Mc (_muticus calvatus_)
| |
FMSA-+ | +--Ss (_spinifer spinifer_)
| | +------+
| | | +--Sh (_spinifer hartwegi_)
| | +--Ssha-+
+--MSA-+ | |
| | +---------Sa (_spinifer asper_)
| |
| +--S---+ +--Sp (_spinifer pallidus_)
| | | +--Spg-+
| | | | +--Sg (_spinifer guadalupensis_)
+--SA--+ +--Sepg-+
| |
| +--Se-----Se (_spinifer emoryi_)
|
+--A---------------------A (_ater_)

--------------------------------------------------------------------------

An arid environment in the central and southern United States and northern Mexico may have increased in area especially southward from Miocene times into the Pliocene (Dorf, 1959:189, 191). The combination of physiographic changes and aridity, which modified the mesic, essentially continuous, aquatic habitats, may have isolated and aided in the differentiation of the _ferox_, _muticus_ and _spinifer_ stocks. Encroachment of the Eocene seas, the maximal extent of which corresponded to the Gulf Coastal Plain and included a northerly extension as far as Cairo in southern Illinois (Mississippi embayment), possibly was an initial barrier isolating the _ferox_ stock of the east.

In the late Miocene or early Pliocene, the MSA (_muticus-spinifer-ater_) stock presumably occupied a large region of the central United States, which extended southward into northern México and along the Gulf Coast at least as far as Alabama. Farther eastward, the _ferox_ stock was isolated in more mesic, probably swampy, marshy habitats.

Later, in the southwestern part of the range of the MSA stock (southern Texas and northern México), the SA and _muticus_ stocks were separated. The _muticus_ stock occurred to the northeastward, and presumably no farther south than the area included within the present drainage basin of the Colorado River. Southward, the SA stock was isolated into several populations that are today represented by _ater_ and _T. s. emoryi_, the most variable subspecies; the distribution of the most distinctive population of _emoryi_ indicates a former isolated inland drainage. The multiple fragmentation of the SA stock presumably terminated by the end of the Pliocene. The progenitors of _T. ater_ probably closely resembled _ferox_. _Trionyx ater_ and _T. ferox_ resemble each other morphologically and in habitat. Therefore, the progenitors of _ater_ are considered to have undergone comparatively little differentiation.

The _spinifer_ stock, occurring principally in the area included within the present drainage basin of the Río Grande, extended its geographic range eastward and became sympatric with _muticus_ and _ferox_. An expansion of range necessarily demands more mesic conditions; these were perhaps afforded by the pluvials (wet, rainy ages) that were coincident with the glacial periods in the Pleistocene (Antevs, 1948:168). The pluvials permitted the isolated populations of the _spinifer_ stock to unite, and permitted that stock to extend its range eastward. The concurrent continental glaciation permitted the _spinifer_ stock to extend its range eastward only in a belt approximately 300 miles wide along the Gulf Coast, and also displaced the ranges of _ferox_ and _muticus_ to southern latitudes. Perhaps _ferox_ was less tolerant of decreased temperatures or changes in habitat than was the _spinifer_ stock but, for some unknown reason, _ferox_ did not extend its range westward. Because _T. ater_ closely resembles _T. s. emoryi_, continued isolation of _ater_ since the beginning of the Pleistocene seems unlikely and _ater_ may have been reunited in subsequent pluvial periods with the _spinifer_ (_emoryi_) stock. A climatic fluctuation between relatively wet and dry periods is corroborated by studies of soil profiles in Trans-Pecos Texas (Bryan and Albritton, 1943).

The separation of the range of _spinifer_ in the general region of western Louisiana, resulting in the differentiation of the _spinifer_ group of subspecies to the east and the _emoryi_ group of subspecies to the west, and the differentiation of _T. s. asper_ and _T. m. calvatus_, both having corresponding western limits of distribution (Mississippi River drainage), are associated with the activities of the Mississippi River and its flood-plain. The combined effects of the pluvials and interpluvials seem responsible for changes in the lower Mississippi Valley. Great volumes of summer melt-water in the glacial stages greatly increased the breadth of the channel of the lower Mississippi River (corresponding to the northern extent of the Mississippi embayment; Hobbs, 1950), and this, coupled with the encroachment of Pleistocene seas (especially in the Mississippi embayment) in the interglacial periods, perhaps separated populations eastward represented today by _T. m. calvatus_ and _T. s. asper_. The _spinifer-hartwegi_ stock probably developed in southern Louisiana in association with the meandering of the Mississippi River and its tributaries, and its broad alluvial plain. The biota of that plain differed from that adjacent to the east or west (see discussion in Viosca, 1944) and constituted a barrier, of a sort, to free communication between the east and west. Westward the _emoryi_ group of subspecies differentiated, its eastern limit probably being the Red River, which followed its own course to the Gulf along the lowlands on the west side of the Mississippi Valley and did not empty directly into the Mississippi until Recent times (Holland, 1944:20). There was not an equally-marked, corresponding separation of the range of _muticus_. However, the juvenal pattern of the subspecies _muticus_ that inhabits the Gulf Coast streams is slightly different (having less short lines) from that of _muticus_ elsewhere.

The Río Grande (inhabited by _emoryi_) presumably had its own exit to the Gulf whereas rivers westward to (and including) the Red River (inhabited by _pallidus-guadalupensis_ cline) probably were joined near their mouths forming a large drainage system. Hubbs (1957:93) pointed out that the Río Grande-Nueces divide also limits a large number of species of fish. The differentiation of _pallidus_ and _guadalupensis_ is possibly due to a difference in the salt content of waters that drain the Edward's Plateau (see page 547), or to isolation of those subspecies in separate drainage systems that had their own exits to the Gulf.

In the lower Mississippi drainage, the _spinifer-hartwegi_ stock extended its range northward following the retreat of the last glacial stage, and differentiated into those two subspecies in the upper Mississippi drainage and Great Lakes-St. Lawrence drainage system.

I have seen one specimen (UMMZ 59198) from the eastern part of the Tennessee drainage (inhabited by _T. s. spinifer_) that resembles _T. s. asper_ (occupying the Gulf Coast drainages of the southeast). This resemblance tends to support the thesis of a former confluence of the Coosa (Alabama River system) and Tennessee drainages as believed by some malacologists to explain resemblances in molluscan fauna and as corroborated by physiographical evidence (see discussion in van der Schalie, 1945).

The Importance of the Study of Turtle Populations in Relation to the History of River Systems

In the Río Grande drainage the geographic distribution of the population of _emoryi_ having orange color in males is approximately the same as that of _Pseudemys scripta gaigeae_; the corresponding distributions suggest that a part of the Río Grande drainage consisting of the Río Conchos in Chihuahua and the Big Bend region of Texas was isolated in former times. Accordingly, the known aquatic chelonian fauna in the basin of Cuatro Ciénegas in central Coahuila, México, is endemic (except _T. s. emoryi_). And the coincidence of the geographic ranges of _T. muticus calvatus_ and _Graptemys pulchra_ in the southeast suggest a former association of the included (Pearl to Escambia) river systems. The occurrence of _T. s. pallidus_ in the Red River drainage indicates that the Red River was formerly associated with the Gulf Coast streams of eastern Texas and western Louisiana (inhabited by _pallidus_) and not with the Mississippi River drainage. The lower Mississippi River valley forms a prominent barrier to the eastern and western dispersal of many kinds of species and subspecies of turtles. _T. m. calvatus_ and _T. s. asper_, which occur in rivers of the Gulf Coast drainage east of the Mississippi, are well-differentiated subspecies showing little or no evidence of intergradation with their relatives in the Mississippi River. The large faunal break provided by the Mississippi River would seem to indicate greater age for that river than for other rivers of the Gulf Coast drainage.

A comparison of the distributions of _Trionyx_ and _Graptemys_ in Texas suggests a faunal break between the drainage systems of the Brazos and Colorado rivers. _Graptemys versa_ occurs in the Colorado and Guadalupe-San Antonio drainages. To my knowledge _versa_ hitherto has not been recorded from the latter drainage system. I have seen one specimen of _Graptemys_ (custody of Gerald Raun, University of Texas) from the Guadalupe River drainage, which I judge to be representative of _versa_, and Olson (1959:48) has reported _Graptemys_ (probably _versa_) in the San Antonio River. The distribution of _G. versa_ parallels in a general way, the distribution of _T. s. guadalupensis_. _G. kohni_ and _T. s. pallidus_ occur in the Brazos River and eastward. Also, it is notable that the population of _T. m. muticus_ occurring in the Colorado River drainage differs slightly (more black pigmentation) from the same subspecies in the adjacent Brazos River system.

There is much difference in the patterns of distribution and degree of differentiation of different genera of aquatic turtles in the eastern United States. Tinkle (1958:41-43, Figs. 49-55) concluded that a general resemblance in the patterns of distribution of the different genera of turtles was evidence that the rates of evolution were essentially the same, assuming that each genus had had a similar time interval for differentiation (_op. cit._:42). If this is true, corresponding patterns of distribution might indicate the same relative age of the population of turtles concerned. Generally, the genera of turtles that on morphological grounds are considered the oldest and most primitive (_Macroclemys_, _Chelydra_) show less differentiation into species and subspecies than those considered younger and more recently evolved (_Graptemys_, _Pseudemys_). In the genus _Graptemys_, much differentiation occurs in the geologically, recently formed, Gulf Coast drainage systems of the southeastern United States. It would seem then, that faster rates of differentiation denote more recent genera, whereas older genera are endowed with a "genetic senility" and are less subject to change.

Evidence of the relative age of two genera of turtles, as suggested by their degree of differentiation into minor taxa, and the degree of difference between populations of two genera that inhabit adjacent drainage systems, may indicate the relative ages of particular river systems. For example, the slight resemblance of _G. versa_ to _kohni_ and the close resemblance of _T. s. guadalupensis_ to _pallidus_ in Texas may reflect the age of the genus _Trionyx_ and the youth of the genus _Graptemys_. Remembering that the genus _Graptemys_ is relatively recently evolved and assuming _G. versa_ to be the most primitive and ancestral species of the genus (at least it is monotypic, the most aberrant species, and unlike any other species of the genus), it seems logical to suppose that the physiographic changes responsible for the Colorado-Brazos divide and the isolation of _versa_ occurred early in the evolutionary history of the genus _Graptemys_. The degree of differentiation of _Trionyx_ suggests that that genus is, comparatively, much older, and that the same physiographic changes responsible for the Colorado-Brazos divide and differentiation of the subspecies _pallidus_ and _guadalupensis_ occurred late in the evolutionary history of the genus _Trionyx_.

In general, patterns of distribution of turtle populations support physiographic evidence concerning changes in stream confluence and relative age of river systems.

SUMMARY

In North America, soft-shelled turtles (genus _Trionyx_) occur in northern México, the eastern two-thirds of the United States, and extreme southeastern Canada. The genus fits the well-known Sino-American distributional pattern. In North America there are four species. Three (_ferox_, _spinifer_ and _muticus_) are well-differentiated and one (_ater_) is not well-differentiated from _spinifer_. Characters of taxonomic worth are provided by the following: size; proportions of snout, head and shell; pattern on carapace, snout, side of head, and limbs; tuberculation; sizes of parts of skull; number of parts of carapaces; and, shape and number of some parts of plastra. Many features show geographical gradients or clines. _T. ferox_ is the largest species and _muticus_ is the smallest. Females of all species are larger than males. With increasing size of individual, the juvenal pattern is replaced by a mottled and blotched pattern in females of all species; adult males of _spinifer_ retain a conspicuous juvenal pattern, whereas the juvenal pattern is sometimes obscured or lost on those of _ferox_ and _muticus_. The elongation of the preanal region in all males, and the acquisition of a "sandpapery" carapace in males of _spinifer_ occur at sexual maturity. There is a marked secondary sexual difference in coloration in a population of _T. s. emoryi_ (side of head bright orange in males and yellow in females). The sex of many hatchlings of _T. s. asper_ can be distinguished by the pattern on the carapace. Slight ontogenetic variation occurs in some proportional measurements. Large skulls of _ferox_ and some _asper_ (those in Atlantic Coast drainages) have expanded crushing surfaces on the jaws. Considering osteological characters, _muticus_ is most distinct; there is less difference between _ferox_ and _spinifer_ than between those species and _muticus_.

_T. ferox_ is monotypic, confined to the southeastern United States, and resembles Old World softshells more than it does any American species. The northern part of the geographic range of _ferox_ overlaps that of _T. s. asper_; there, the two species are ecologically isolated. _T. spinifer_ is polytypic, has the largest geographic range, and is composed of six subspecies, of which two are described as new (_pallidus_ and _guadalupensis_). The subspecies are divisible into two groups. One, the _spinifer_ group (_spinifer_, _hartwegi_ and _asper_) is recognized by a juvenal pattern having black spots or ocelli; _asper_ is the most distinctive and shows little evidence of intergradation in the lower Mississippi River drainage with the _spinifer-hartwegi_ complex, which, northward, is differentiated into two subspecies in which there is an east-west cline in size of the ocelli on the carapace. The _emoryi_ group (_pallidus_, _guadalupensis_, _emoryi_) is recognized by a pattern of white spots; _emoryi_ is most distinctive. Each of several characters behaves as a cline if traced from east to west through the three subspecies. _T. s. pallidus_ intergrades with the _spinifer-hartwegi_ complex in the lower Mississippi River drainage. _T. s. emoryi_ is the most variable subspecies; in its most notable population the males have orange coloration. _T. s. emoryi_ has been introduced into the Colorado River drainage of Arizona. _T. ater_ most closely resembles _T. s. emoryi_, but shows alliance with _T. muticus_ and _T. ferox_. _T. ater_ is confined to ponds of crystal-clear water in central Coahuila, México. _T. muticus_ is completely sympatric with _spinifer_, and is composed of two subspecies (_muticus_ and _calvatus_). _T. m. calvatus_ shows no evidence of intergradation in the lower Mississippi River drainage with _T. m. muticus_, corresponding somewhat to the relationship of _T. s. asper_ with the intergradient population of _T. spinifer_ in the Mississippi River.

Softshells have pharyngeal respiration and probably are incapacitated by rotenone. _T. ferox_ and the subspecies of _spinifer_ occur in a wide variety of fresh-water habitats; _muticus_ is more nearly restricted to running water (especially in the northern parts of its range) than _spinifer_, and may be less vagile than _spinifer_. _T. ferox_ is more tolerant of marine and brackish waters than are _muticus_ or _spinifer_. Small size and pallid coloration seem correlated with arid environments. The largest species (_ferox_) and the smallest population of _spinifer_ (resembling _muticus_) both occur in the southernmost part of the range of the genus. Diurnal habits include basking on shores or débris in water, floating at the surface, procuring food, and burrowing in shallow and deep water (no observations for _spinifer_ and _muticus_ in deep water). Softshells are principally carnivorous; the food consists mostly of crawfish and insects; there is evidence of cannibalism involving predation on first- and second-year-old turtles. The capture of food is triggered primarily by movement of prey; sight seems to be more important than smell to _Trionyx_ in capturing food. There is no indication of a food preference between species; enlarged crushing surfaces of jaws in some _ferox_ and _asper_ may be an adaptation for feeding on mollusks. Schools of fish are reported to follow softshells, and presumably acquire food that is dislodged by the grubbing and scurrying of the turtles on the bottom. Softshells are wary. They are good swimmers, and travel rapidly on land. The depressed body is an adaptation for burrowing and concealment. Permanent growths of algae do not occur on the dorsal surface of softshells. There is evidence of some nocturnal activity, and a general parallel in habits between trionychids and chelydrids. Softshells sometimes move overland; they move little in aquatic habitats. The normal annual period of activity of _spinifer_ in latitudes 40° to 43° is approximately five months from April into September, depending on the weather; they hibernate under a shallow covering of mud in deep water. The southernmost populations may be active throughout the year.