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).
[Illustration: FIG. 3. Ratio of length of plastron to width of head
(PL/HW) in some American species and subspecies of the genus
_Trionyx_. The size of each sample is given in parentheses
following an indication of the range (< = less than, > = greater
than) in length of plastron (in cm.) of each sample. The horizontal
line indicates the observed variation; the vertical line, the mean;
the white rectangle, four standard deviations; and the black
rectangle, four standard errors of the mean. There is some
ontogenetic variation in PL/HW. The head is narrowest in _muticus_
and widest in _ferox_.]
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").
[Illustration: FIG. 4. Ratio of length of carapace to width of
carapace (CL/CW) in some American species and subspecies of the
genus _Trionyx_. Symbols as in Fig. 3. There is some ontogenetic
variation in CL/CW (least in _muticus_). The carapace is
narrowest in _ferox_ and _emoryi_, and widest in _muticus_,
_pallidus_ and _asper_.]
[Illustration: FIG. 5. Pattern on dorsal surface of snout of some
American species and subspecies of the genus _Trionyx_. Note the
gradual transition in pattern from that of _hartwegi_ (b) and
_asper_ (c) to that of _emoryi_ (h).
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.
[Illustration: FIG. 6. Pattern on side of head of some American
species and subspecies of the genus _Trionyx_. Note the gradual
reduction in contrast of pattern and interruption of the postocular
stripe from that of _spinifer_ (b) to that of _emoryi_ (f).
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)
]
[Illustration: FIG. 7. Pattern on the dorsal surface of the distal
part of the right hind limb of some American species and subspecies
of the genus _Trionyx_. Note the gradual reduction in contrast of
pattern from that of _hartwegi_ (a) to that of _emoryi_ (d).
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)
]
[Illustration: FIG. 8. Shape of tubercles on anterior edge of
carapace in some American species and subspecies of the genus
_Trionyx_ (× 1/2). Note the gradual reduction in size of
tubercles from that of _hartwegi_ (b) to that of _muticus_ (h).
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.
[Illustration: FIG. 9. Anteroposterior position of plane of
greatest width of carapace (CL/PCW) in some American species
and subspecies of the genus _Trionyx_. Symbols as in Fig. 3.
The greatest width of carapace is midway between anterior and
posterior ends in _ferox_, _spinifer_, _hartwegi_, _asper_ and
_muticus_, and farther posterior in the other subspecies of
_spinifer_.]
_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_.
[Illustration: FIG. 10. Ratio of width of head to length of snout
(HW/SL) in some American species and subspecies of the genus
_Trionyx_. Symbols as in Fig. 3. Values for _spinifer_ are
combined with those of _hartwegi_, and those of _pallidus_
with _guadalupensis_. The snout is proportionately the
longest in _muticus_.]
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.
[Illustration: FIG. 11. Ratio of diameter of ocellus to length of
plastron (OD/PL) in _T. spinifer_ from some states in the upper
Mississippi River and Great Lakes drainages. Symbols as in
Fig. 3. The size of the ocelli on the carapace gradually
increases from Kansas northeastward to Michigan.]
Character Analysis
_Snout_
The snout (Fig. 12) is tubate having terminal nostrils separated by a
vertical septum. One of the principal characters distinguishing _T.
ferox_ and _T. spinifer_ from _T. muticus_ is a lateral, whitish ridge
projecting from each side of the nasal septum (hereafter referred to
as septal ridges but often referred to in the literature as a
papilla). The shape of the end of the snout is truncate in _T. ferox_
and _T. spinifer_, and the nostrils are larger than in _T. muticus_.
In _muticus_ the snout usually terminates somewhat obliquely, and the
nostrils tend to be slightly inferior; also, the end of the snout is
usually rounded and somewhat pointed, causing the nostrils to be
visible in lateral view. Some _T. muticus_ do not differ markedly from
_ferox_ or _spinifer_ in shape of the end of the snout. Stejneger
(1944:14) mentioned indication of a septal ridge that did not reach
the opening of the nostril in _muticus_. I have slit the outer edge of
the nostril on several specimens of _muticus_, and have not noticed an
indication of a septal ridge.
[Illustration: FIG. 12. Shape of snout in _T. spinifer_ (left, a-d,
from KU 46907) and _T. muticus_ (right, e-h, from KU 48236).
Lateral views--a, e (× 1); anterior views--b, f (× 5); dorsal
views--c, g (× 2.5); ventral views--d, h (× 2.5).]
_Tuberculation_
Tubercles or obtuse prominences occur on the anterior edge of the
carapace (Fig. 8) or on the dorsal surface of the carapace. _Trionyx
muticus_ lacks tubercles, although some individuals show shallow,
widely spaced wrinkles that suggest prominences on the anterior edge
of the carapace. Both sexes of _T. ferox_ have prominences, resembling
flattened hemispheres, on the anterior edge of the carapace and in the
nuchal region. Large females of _ferox_ have obtuse prominences in the
center of the carapace posteriorly, some of which are often arranged
in longitudinal rows. The surface of the carapace in both sexes of _T.
ferox_ has small closely-set, blunt tubercles arranged in rows that
resemble longitudinal ridges (most evident in juveniles).
Large females of _T. spinifer_ have obtuse prominences in the center
of the carapace posteriorly, some of which in many specimens are
arranged in longitudinal rows; I cannot discern any correlation of
number or arrangement of prominences with size in _spinifer_ or
_ferox_. The carapace in adult males of _spinifer_ bears small, sharp
tubercles that make the surface feel like sandpaper. The tubercles on
the anterior edge of the carapace in adults of both sexes vary from
round to equilateral and conical to low and flattened (see comments on
tuberculation under subsection entitled "Geographic Variation"). Some
large females of the same subspecies have tubercles on the anterior
edge of the carapace that may be conical (higher than wide) or
equilateral. The difference in shape of the tubercles seems not to be
correlated with size because one _T. s. pallidus_, 30.5 centimeters
(TU 13212) has prominent but blunted and equilateral tubercles,
whereas, another female of _pallidus_, 20.8 centimeters (TU 13210),
from the same locality has higher, conical tubercles. The blunted,
equilateral tubercles may be the result of environmental wear, or the
difference in shape of tubercles may be due to individual variation.
_Pattern on Carapace_
Two features of the pattern on the carapace are of taxonomic worth: 1)
the width and distinctness of the pale rim at the periphery of the
carapace (marginal rim), if present, and 2) the kind of pattern on the
carapace (juvenal pattern). The marginal rim is absent in females of
_T. ater_, and only faintly evident in males. The marginal rim is
obscured or absent (adult males and females) and is not separated from
the ground color of the carapace by a dark marginal line in hatchlings
of _T. ferox_. The carapace of _T. muticus_ has a marginal rim that is
usually separated from the ground color of the carapace by an
ill-defined, dark marginal line; some individuals lack the marginal
dark line. The subspecies of _T. spinifer_ have a well-defined, dark,
marginal line that separates the marginal rim from the ground color of
the carapace; _T. s. asper_ has more than one dark marginal line on
the carapace. The marginal rim is ill-defined and blotched, or absent,
in large females of all species of _Trionyx_.
The marginal rim is widest at the posterior end of the carapace and
lacking in the nuchal area. The width of the pale marginal rim is very
narrow, almost to the degree of being absent, in juveniles of _T.
ferox_. _T. s. emoryi_ has a pale, marginal rim that is four or five
times wider posteriorly than it is laterally, whereas posteriorly the
width of the rim in the other subspecies of _T. spinifer_ and in the
species _T. muticus_ is only two or three times wider posteriorly than
it is laterally.
The juvenal pattern commonly consists of whitish tubercles or dots
(_T. s. emoryi_, _T. s. guadalupensis_, _T. s. pallidus_, _T. ater_),
large black ocelli (_T. s. spinifer_), small black dots and ocelli
(_T. s. hartwegi_, _T. s. asper_), large dusky spots or ocelli (_T. m.
calvatus_), or small dusky dots or short streaks and dashes (_T. m.
muticus_). Some hatchlings of _pallidus_ and _emoryi_ have a uniform
pale brown or tan carapace; hatchlings of _T. ferox_ have a
distinctive pattern (Pl. 31). Further comments and illustrations
pertaining to kind of pattern on the carapace are offered under the
accounts of species and subspecies.
_Pattern on Dorsal Surface of Snout (Fig. 5)_
_T. ferox_ has pale stripes on a dark background that unite in front
of the eyes; the dark ground color becomes paler with increasing size,
but the stripes retain thick black borders. _T. m. muticus_ has
ill-defined, pale stripes that are evident just in front of the eyes
and do not extend anteriorly to unite in front of the eyes, whereas
_T. m. calvatus_ lacks pale stripes on the snout. The kind of pattern
on the dorsal surface of the snout that is characteristic for each of
the subspecies of _T. spinifer_ has been mentioned in the discussion
of clinal variation.
_Pattern on Side of Head (Fig. 6)_
_T. ferox_ has a pale broad, postocular stripe in contact with the
orbit or not, and other pale marks on a dark background; the ground
color becomes paler with increasing size, but the stripes and other
marks retain thick black borders. _T. m. muticus_ usually has an
uninterrupted, dusky-bordered, postocular stripe, whereas _T. m.
calvatus_ (in adult males only) has pale postocular stripes with thick
blackish borders. The pattern on the side of head that is
characteristic for each subspecies of _T. spinifer_ has been mentioned
in the discussion of clinal variation.
_Pattern on Dorsal Surface of Limbs (Fig. 7)_
Young specimens of _T. ferox_ have pale marks on a blackish
background. As growth proceeds the distinctive contrasting pattern is
obliterated and eventually is replaced by a uniform grayish coloration
in large adults. The pattern on the limbs of _T. muticus_ is not
contrasting, and is almost a uniform grayish, consisting of fine, pale
markings. The clinal variation in pattern and kind of pattern on the
limbs of the subspecies of _T. spinifer_ has been mentioned in the
discussion of clinal variation. Dark markings tend to form streaks
that are coincident with the digits, and larger markings occur on the
hind limbs than on the forelimbs.
_Marginal Ridge_