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 [465] 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.
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.