North American Recent Soft-shelled Turtles (Family Trionychidae) Part 4

_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_

The anterolateral edge of the carapace in _T. ferox_ (both s.e.xes and 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 "Hautsaume" 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_.

[Ill.u.s.tration: 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 p.r.o.nounced, cusplike, free edge.

[Ill.u.s.tration: 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).]

_Choa.n.a.l 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 softsh.e.l.ls, and Parsons (1958) discussed their occurrence in sea turtles of the family Cheloniidae and in the testudinid subfamily Emydinae (1960). In preserved softsh.e.l.ls the choa.n.a.l 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, choa.n.a.l papillae occur in all American species and subspecies of soft-sh.e.l.led 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 choa.n.a.l 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 aga.s.sizi_ (= _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 s.e.xual differences between skulls of corresponding size, except in _aga.s.sizi_-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).

[Ill.u.s.tration: 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 softsh.e.l.ls as described by Loveridge and Williams (_op. cit._:414, 429, fig. 54) for _Lissemys_, _Cyclanorbis_, _Cycloderma_ and some _Trionyx triunguis_.