all sizes) is "folded over" into a ridge having a distinct inner
margin (Pls. 1 and 2), which is hereafter referred to as the marginal
ridge. Siebenrock (1924:184-85) referred to this ridge as a
"Hautsäume" and mentioned its occurrence in Old World species of the
genus _Trionyx_. The marginal ridge is not present in _T. muticus_,
_T. spinifer_ or _T. ater_.
_Ratios_
The means of some samples (Fig. 3) differ in regard to PL/HW, but the
ranges of variation overlap so much that little significance can be
attributed to the difference. _T. ferox_, and to a lesser extent _T.
s. emoryi_ and _T. s. asper_, have slightly larger heads than the
other forms. The width of head is proportionately the smallest in _T.
muticus_; in most individuals of it having a plastron so long as 13.0
centimeters, the width of the head is less than 16 per cent of the
length of the plastron--a percentage that is distinctive.
The visibly narrower carapace (CL/CW, Fig. 4), suggesting an ovoid or
oblong shape, in some large individuals of _T. ferox_ and _T. s.
emoryi_ is indicated by the large ratio in specimens that have a
plastral length of 8.0 centimeters or more. Nevertheless, the degree
of overlap of the ranges of variation is such that this ratio is of
relatively little use taxonomically.
The greatest width of the carapace is farther posterior in _T. s.
emoryi_ than in the other forms (CL/PCW, Fig. 9). The considerable
overlap of the range of variation of this ratio for _emoryi_ with the
other forms limits its usefulness as a taxonomic character.
The snout is proportionately shortest in _ferox_ and _T. s. emoryi_,
and longest in _muticus_ (HW/SL, Fig. 10). The most marked difference
in this ratio is between the species _muticus_ and _ferox_; the ranges
of variation of those species overlap to a degree that tends to negate
the taxonomic usefulness of this character.
Most adults and subadults of _T. ferox_ show clearly in dorsal view
the anterolateral portions of the plastron. This condition is much
less well developed in some specimens of _T. s. emoryi_. _T. ferox_ is
extreme in the ratio CL/PL (relatively the longest plastron or
shortest carapace, Fig. 13). _T. s. asper_ has the shortest plastron
in relation to length of carapace. Calculated ratios for 12 _T. ater_
average 1.36, a value that suggests close affinity with some
subspecies of _T. spinifer_ (_pallidus_, _guadalupensis_, _emoryi_).
Because of the degree of overlap of the ranges of variation in all
forms, little significance can be attributed to the difference in
means of _ferox_ and _asper_.
[Illustration: FIG. 13. Ratio of length of carapace to length of
plastron (CL/PL) in some American species and subspecies of
the genus _Trionyx_. Symbols as in Fig. 3. _T. ferox_ has
proportionately the shortest carapace.]
_Scalation_
Cornified, smooth or cusplike areas occur on each limb, but their
number and arrangement are of no taxonomic value. Normally, the
anterior surface of each forelimb possesses four cornified areas for
which the term antebrachial scales is proposed (Fig. 14). Two of the
four scales occur in a more dorsal position; the lateral edge of the
proximal one is free and cusplike along a part of its length, whereas
the distal scale is smooth-edged. Two scales having their lateral
edges free and cusplike are ventral in position, and closer together
than the two dorsad scales. Size of the scales and length of the free
cusplike edges vary. Occasionally adjacent scales are fused or small
additional scales are present. The number, configuration and
arrangement of the two cornified areas on each hind limb are constant.
One of these scales is smooth-edged and occurs posteriorly on the
dorsal surface. The other scale, situated on the ventral surface
posteriorly in the region of the heel and distal to the smooth-edged
scale of the dorsal surface, has a pronounced, cusplike, free edge.
[Illustration: FIG. 14. Dorsal surface of right forelimb showing
normal number and arrangement of antebrachial scales in American
species of the genus _Trionyx_ (_T. spinifer hartwegi_,
KU 15932, × 3/4).]
_Choanal Papillae_
This term refers to the papillate flaps of skin that project from the
lateral borders of the internal nares. Webb and Legler (1960:23) noted
their presence in softshells, and Parsons (1958) discussed their
occurrence in sea turtles of the family Cheloniidae and in the
testudinid subfamily Emydinae (1960). In preserved softshells the
choanal papillae may extend laterally and partly cover the nares, or
may be folded vertically against the lateral borders of the nares; in
the latter position the papillae are easily overlooked. To my
knowledge, choanal papillae occur in all American species and
subspecies of soft-shelled turtles. The free edge of each narial flap
shows various degrees of fimbriation. The fimbriated border is least
developed (margin nearly entire) in _T. muticus_ and most developed in
_T. ater_ and _T. ferox_. In _ater_ at least, the anteriormost
portions of the narial flaps seem wider than in the other forms and
show a greater degree of fimbriation than the posteriormost parts. The
choanal papillae are most easily observed in large specimens.
_Skull_
In general, there is less difference between the skulls of _ferox_ and
_spinifer_ than between either of those species and _muticus_
(Stejneger, 1944:10-11). Figure 15 shows the general differences in
proportions of the skulls of _spinifer_ and _muticus_; Plate 54 shows
the skull of the holotype of _Platypeltis agassizi_ (= _T. s. asper_),
which is similar to that of _ferox_; Stejneger (_op. cit._) provided
labelled drawings of the skull of _T. spinifer_ as well as photographs
of skulls of other forms.
The total of 159 skulls examined by me include 80 of _spinifer_, 50 of
_ferox_, and 29 of _muticus_. There are no secondary sexual
differences between skulls of corresponding size, except in
_agassizi_-form skulls mentioned under the account of _T. s. asper_,
and possibly in _ferox_. Most, and possibly all, of the skulls of
_muticus_ having a basicranial length of 40.0 millimeters or more, and
those of _spinifer_ exceeding 50.0 millimeters must represent females
(by correlation of known maximum size of males with greatest width of
head, which is, in turn, compared with the greatest width of skull and
corresponding basicranial length).
[Illustration: FIG. 15. Skulls of _Trionyx spinifer hartwegi_ (left,
a-d, KU 2757), and _Trionyx muticus muticus_ (right, e-h, KU 1870).
Dorsal views, a (× 1/2), e (× 3/4); occipital views, b (× 5/6),
f (× 1); lateral views, c (× 1/2), g (× 3/4); ventral views,
d (× 1/2), h (× 3/4).
a., alveolar surface of upper jaw
aq., articular surface of quadrate
ex., exoccipital
fp., fenestra postotica
fm., foramen magnum
if., intermaxillary foramen
ic., internal choana
mx., maxilla
mxb., maxillary bridge
oc., occipital condyle
op., opisthotic
ope., opisthotic-exoccipital spur
opw., opisthotic wing
pmx., premaxillaries (fused)
pt., pterygoid
q., quadrate
qj., quadratojugal
sq., squamosal
s., supraoccipital spine
tc., tympanic cavity
]
Measurements used include basicranial length (occipital condyle to tip
of upper jaw), greatest width (variable in position), greatest width of
alveolar surface of maxilla (taken at level immediately posterior to
anterior margin of internal choanae), greatest length of internal
choanae, and least breadth of maxillary bridge (separating internal
choanae and intermaxillary foramen). One ratio developed from the
measurements was greatest length of internal choanae/least breadth of
maxillary bridge, hereafter referred to as IC/MB. This ratio is
discussed under the account of _T. s. asper_.
_Greatest Width_
The position or level on the skull where the greatest width (Table 3)
occurs is of some diagnostic value in distinguishing the skulls of
_ferox_ from _spinifer_ and _muticus_. Skulls of _ferox_ usually are
widest at the level of the quadratojugal (immediately in front of
tympanic cavity), whereas skulls of _spinifer_ and _muticus_ usually
are widest slightly more posteriorly at a level on the squamosal
immediately behind the tympanic cavity. Occasionally the width at the
level of the quadratojugal and squamosal is the same, or the greatest
width of skull may be ventrad between the quadrates, which are
slightly flared laterally. The latter condition possibly is most
prevalent in _muticus_.
TABLE 3. Variation in Position of Greatest Width of Skull of North
American Species of the Genus Trionyx (excluding ater). The Number
of Specimens Examined (in Parentheses) Follow the Specific Names.
================+=================================================
| Species
POSITION +--------------+-----------------+----------------
| _ferox_ (36) | _spinifer_ (47) | _muticus_ (14)
----------------+--------------+-----------------+----------------
Squamosal | 7 (19%) | 35 (74%) | 11 (79%)
Quadratojugal | 26 (72%) | 7 (15%) | 1 (7%)
Quadrate | 2 (6%) | | 2 (14%)
Squamosal and | | |
quadratojugal | | |
of same width | 1 (3%) | 5 (11%) |
----------------+--------------+-----------------+----------------
_Supraoccipital Spine_
The ventral surface of the supraoccipital spine in _muticus_ lacks a
medial ridge, and gradually increases in width anteriorly, so that it
is widest proximally in the region of the roof of the foramen magnum.
In _ferox_ and _spinifer_, the ventral surface, usually having a
medial ridge, is narrow and of the same width throughout its length or
somewhat flared distally. The ventral surface of the supraoccipital
spine, which is widest proximally in _muticus_, is always narrow
proximally in _ferox_ and _spinifer_. The ventral surface of the
supraoccipital spine of one skull of _spinifer_, USNM 91311, differs
little from that of _muticus_.
_Foramen Magnum_
The shape of the foramen magnum is generally rhomboidal in _spinifer_
and _ferox_; the ventral angle is semicircular, the lateral angles
obtuse, and the dorsal angle more acute. The shape of the foramen
magnum in _muticus_ is ovoid, higher than wide; the sides are evenly
rounded.
_Opisthotic-Exoccipital Spur_
Skulls of _spinifer_ normally have the fenestra postotica partly
restricted by a medially-slanting, descending spur from the roof of
the fenestra postotica; the spur incorporates the suture between the
exoccipital and opisthotic and includes parts of those two bones. On
one skull (KU 2824) the spur is displaced more medially and does not
incorporate the opisthotic. The descending spur contacts the pterygoid
ventrally forming a complete bony strut traversing the fenestra
postotica in some skulls (KU 2228, 2666, 2762, TU 15423, MCZ 46621, TU
15415, right side only). The fenestra postotica on skulls of _ferox_
and especially _muticus_ is not normally restricted by an
opisthotic-exoccipital spur.
Often the spur is reduced and indicated by a smooth projecting ridge.
Sometimes the spur or ridge is absent on skulls of _spinifer_, and I
have seen no well-developed spur on a skull of _muticus_. The
development of the spur is not due to ontogenetic variation. There is
some variation in development of the spur on either side of the skull;
two skulls of _ferox_ have the combination ridge/absent, and two of
_spinifer_ have the combinations ridge/spur and spur/absent. The
frequency (based on counts of individual skulls) and the degree of
development of the spur among the three species is indicated in Table
4.
TABLE 4. Frequency and Degree of Development of Opisthotic
Exoccipital Spur of North American Species of the Genus Trionyx
(excluding ater). The Number of Specimens Examined (in Parentheses)
Follow the Specific Names.
======================+=================================================
| Species
DEVELOPMENT OF SPUR +--------------+-----------------+----------------
| _ferox_ (43) | _spinifer_ (68) | _muticus_ (29)
----------------------+--------------+-----------------+----------------
spur (well-developed) | 1 (2%) | 45 (66%) |
ridge (reduced) | 7 (16%) | 20 (30%) | 1 (3%)
absent | 35 (82%) | 3 (4%) | 28 (97%)
----------------------+--------------+-----------------+----------------
Loveridge and Williams (1957:415, footnote) cited Siebenrock who
mentioned a descending process of the opisthotic in _Dogania_ (=
_Trionyx_) _subplana_ and _Trionyx sinensis_. I have not seen an
ascending process of the pterygoids on skulls of American softshells
as described by Loveridge and Williams (_op. cit._:414, 429, fig. 54)
for _Lissemys_, _Cyclanorbis_, _Cycloderma_ and some _Trionyx
triunguis_.
_Opisthotic Wing_
This term refers to the laterally directed, posterior part of the
opisthotic that is visible in occipital, lateral and ventral views. In
ventral view the opisthotic wing is most easily seen and is wider in
_muticus_ than in _spinifer_ or _ferox_. In _muticus_ the distal part
is truncate, whereas in _ferox_ and _spinifer_, it is more tapered and
gently rounded, although somewhat unevenly flared medially. Also
there is more of a downward curvature (in ventral view) of the
opisthotic wing in _muticus_ than in _ferox_ or _spinifer_;
consequently the tip of the wing in _muticus_ is often just visible in
dorsal view (on lateral side of squamosal), certainly in lateral view.
The distal part or tip of the opisthotic wing is not visible in dorsal
view on skulls of _ferox_ or _spinifer_.
_Articular Surface of Quadrate_
The ventral surface of the quadrate that articulates with the mandible
is composed of a lateral condyle and a medial articular surface. The
condyle and medial articular surface are separated by a furrow. On
skulls of _ferox_ and _spinifer_ the lateral condyle, which is not
conspicuously tapered posteriorly, is slightly larger than the medial
articular surface, and the furrow is shallow. On skulls of _muticus_,
the lateral condyle is conspicuously tapered posteriorly, is slightly
smaller than the medial articular surface, and the furrow is deep.
_Contact of Maxillaries Above Premaxillaries_
The contact of the maxillaries above the premaxillaries is of
diagnostic value in distinguishing skulls of _ferox_ and _spinifer_
from those of _muticus_. I have seen no skulls of _muticus_ on which
the maxillaries were in contact, and no skulls of _ferox_ on which the
maxillaries were separated. Stejneger (1944:19), however, reported a
skull of _muticus_ (USNM 102677) having the maxillaries in contact.
Maxillaries are in contact (sometimes just barely) in 65 of 74 skulls
of _spinifer_ (88%); the premaxillaries are separated on nine skulls
(12%).
_Carapace_
The dorsal surface of the bony carapace of American trionychids
consists of a nuchal, seven or eight pairs of pleurals, and seven or
eight, rarely nine, neurals (Fig. 16). The lateral parts of the nuchal
overlie the second pair of ribs. The distal parts of the second
through the ninth pair of ribs extend laterally beyond the lateral
edges of the pleurals. There are no marginal ossifications. The
posterior part of the bony carapace bears blunt, rounded or ovoid to
linear, prominences mostly on the last pair of pleurals principally on
large females of _spinifer_ and _ferox_; I have seen only one adult
male (stuffed, MCZ 46633) having a semblance of welts on the bony
carapace. The nuchal, pleurals and neurals are sculptured.
As growth proceeds, the single, transversely-oriented, fontanelle of
young turtles that separates the nuchal from the first neural and
first pair of pleurals divides into two fontanelles that generally
decrease in size and finally disappear. Occasionally only one
(unilateral) large fontanelle is present (USNM 54734, _muticus_). The
largest specimens noted that retain fontanelles are a _ferox_ (USNM
029474) having a plastron 24 centimeters long, and a _spinifer_ (USNM
54731) having a plastron 20 centimeters long. The fontanelles probably
are present in some larger individuals.
[Illustration: FIG. 16. Carapace of _Trionyx spinifer_ (a), and
sketches of posterior parts of carapaces (b-i) of three American
species, showing number and variation in arrangement of neurals and
pleurals (not to scale; seventh neural, n7, and pleural, p7).
a. KU 2226, Lewisville, Lafayette County, Arkansas (× 1/3);
sculpturing incompletely shown. Labels: r, ribs; nu, nuchal; n,
neurals 1-7; p, pleurals 1-7.
b. _ferox_, USNM 60496, Auburndale, Polk County, Florida.
c. _muticus_, KU 1964, Doniphan Lake, Doniphan County, Kansas.
d. _spinifer_, USNM 100380, Plaquemine, Iberville Parish,
Louisiana.
e. _muticus_, TCWC 7260, Red River, 8 mi. NW Ringgold, Montague
County, in Clay County, Texas.
f. _spinifer_, USNM 59266, Homer, Winona, Minnesota.
g. _muticus_, KU 2840, White River, DeValls Bluff, Prairie County,
Arkansas.
h. _muticus_, USNM 115939, Mississippi.
i. _muticus_, USNM 54734, Mississippi River, Fairport, Muscatine
County, Iowa.
]
Most variation concerns the number of neurals and pairs of pleurals, and
their arrangement posteriorly (H. M. Smith, 1947:121, table; Stejneger,
1944:18). Table 5 shows the frequency of occurrence of the number of
neurals, pairs of pleurals, and the separation or contact of the seventh
pair of pleurals; figure 16 illustrates some of the configurations of
these plates posteriorly (e, g, and i not included in Table 5). The
eighth pair of pleurals is reduced or absent (Loveridge and Williams,
1957:417). Eight neurals and eight pairs of pleurals occur in all three
species. The seventh pleurals may contact each other in all three
species, and their separation has been observed only in the species
_spinifer_ and _muticus_. Seven neurals and contact of the seventh pair
of pleurals, or eight neurals and separation of the seventh pair of
pleurals from each other occurs with approximately equal frequency in
the species _muticus_. _T. ferox_ and _spinifer_ most often have seven
neurals, seven pairs of pleurals, and the seventh pair of pleurals in
contact. Stejneger (_loc. cit._) mentioned a specimen in MCZ having nine
neurals; I recorded nine neurals for USNM 54734 (Fig. 16i) for which
Stejneger (_loc. cit._) recorded eight. AMNH 57384 (_ferox_) has a small
eighth pleural on the left side only, and USNM 115939 (_muticus_) has an
eighth pleural only on the right side (Fig. 16h). Anomalous conditions
observed included: an accessory bone between the first and second
pleurals on the right side that contacts the first and second neurals in
USNM 54733, (_muticus_); only six neurals in USNM 95193 (_spinifer_); a
small accessory bony element between the first and second neurals in
AMNH 57383 (_ferox_); and, only six pleurals (second and third fused) on
the right side in USNM 54734 (_muticus_).
TABLE 5. Frequency of Occurrence of Number of Neurals, Pairs of
Pleurals, and Separation or Contact of the Seventh Pair of
Pleurals Among Species of American Soft-shell Turtles
===================+=================+==================================
Number | Contact (+) or | Species
--------+----------+ separation (-) +---------+------------+-----------
| Pairs of | of seventh pair | _ferox_ | _spinifer_ | _muticus_
Neurals | pleurals | of pleurals | (16) | (60) | (34)
--------+----------+-----------------+---------+------------+-----------
7 | 7 | + | 9 (56%) | 50 (83%) | 13 (38%)
7 | 8 | + | 5 (31%) | 2 (3%) | 2 (6%)
8 | 7 | + | 2 (13%) | 3 (5%) | 3 (9%)
8 | 8 | + | | 4 (7%) | 2 (6%)
8 | 7 | - | | 1 (2%) | 14 (41%)
--------+----------+-----------------+---------+------------+-----------
Ventrally, the bony carapace shows ten thoracic vertebrae, the second
through the ninth having well-developed, depressed ribs that are fused
(no sutures) to the pleurals. The ribs of the first thoracic vertebra
are represented by bony struts that extend posterolaterally and
contact the anterior borders of the second pair of ribs. The two ribs
of the ninth pair are free for most of their length and often are
broken; they are slightly shorter than the eighth pair of ribs. The
ribs of the tenth thoracic vertebra may be well-developed (KU 2219,
2666, 50856, _spinifer_, and 16528, _ferox_), but are usually broken
off and represented only by transverse processes.
Kyphosis
Kyphosis (angular curvature of the vertebral column) or the
hump-backed condition in American softshell turtles has been
summarized by Nixon and Smith (1949:28). Cahn (1937:185, pl. 25e)
illustrated the condition in an individual of _T. spinifer_, and H.
M. Smith (1947:119) mentioned kyphotic softshells representing the
species _spinifer_ (subspecies _hartwegi_ and _emoryi_) and _muticus_.
Neill (1951:10) mentioned two kyphotic _T. s. asper_ and Nixon and
Smith (_loc. cit._) recorded the report of a kyphotic _T. ferox_. I
have noted the condition in four _muticus_ (subspecies _muticus_, KU
1959-60, 23230; INHS 2148) and seven _spinifer_ (CNHM 22925;
subspecies _hartwegi_, USNM 55689; subspecies _spinifer_, UMMZ 52948,
95615; subspecies _emoryi_, KU 2219, 33523, TU 16240). The smallest
kyphotic specimen, a hatchling, TU 16240, has a plastral length of 3.5
centimeters. Kyphosis is to be expected in all kinds of softshells as
are other abnormalities, such as albinism (reported for _Lissemys_ by
D'Abreu, 1928, and partial albinism noted in _T. cartilagineus_ by
Mohr, 1929) or congenital absence of limbs (reported by Dutta, 1931,
as occurring in the genera _Trionyx_ and _Lissemys_). The cause of
kyphosis is not known. Smith (_op. cit._:120) suggested an abnormally
early fusion of the costals (= pleurals) with the ribs, and a
subsequent differential rate of growth between them and the vertebral
column as a hypothesis; Williams (1957:236) proposed that late
retraction of the yolk mass, or retraction of an excessively large
yolk mass may cause kyphosis. The cause of kyphosis may be of genetic
origin or due to some environmental damage to the vertebral column
prior to the cessation of growth. The variation in rate of growth of
the vertebral column may produce humps of different shapes and sizes.
Some of the specimens noted above (UMMZ 52948, 95615) have the
carapace only slightly arched and are considered partly kyphotic.
There seem to be degrees of kyphosis, a fact that should be taken into
account in considering the occurrence of variation in greatest depth
of shell.
Plastron
The plastron is united to the carapace by ligamentous tissue and is
somewhat flexible anteriorly and posteriorly. Anteriorly the plastron
is somewhat hingelike and may contact the anteriormost edge of the
carapace. The bony elements are reduced. There is usually a median
vacuity, which is relatively smaller in larger specimens and may be
divided into two vacuities (a posteromedial and an anteromedial) by
the medial juxtaposition of the hyo-hypoplastra, especially in
_muticus_. Williams and McDowell (1952) have recommended a change in
nomenclature for some of the plastral bones on the basis of
reinterpretation of their homologies. The nine plastral bones include:
an anterior pair of preplastra (= epiplastra, _auct._); an unpaired,
median bone, representing fused epiplastra (= entoplastron, _auct._),
hereafter referred to as the epiplastron; a pair of hyoplastra; a pair
of hypoplastra; and, posteriorly, a pair of xiphiplastra (Fig. 17).
Siebenrock's (1902) synopsis of living trionychids was based entirely
on plastral characters. He distinguished between _muticus_ and
_spinifer_ principally by the shape of the epiplastron; _T. ferox_ was
not considered different from _spinifer_. The median angle formed by
the boomerang-shaped epiplastron is obtuse and somewhat greater than
90 degrees in _muticus_ (Fig. 17a); the angle of the epiplastron in
_spinifer_ and _ferox_ is smaller than in _muticus_ and forms an
approximate right angle (Fig. 17b). Williams and McDowell (_op.
cit._:277, Pl. 1, Fig. 3) presented an illustration of the anterior
plastral elements of an adult _T. ferox_. Siebenrock provided
illustrations of the plastrons of _muticus_ (_op. cit._:823, Fig. 5)
and _spinifer_ (_op. cit._:830, Fig. 10).