Marginal Ridge
The anterolateral edge of the carapace in T. ferox (both sexes 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 "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 [464] 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.
