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 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_