The alveolar surfaces of the jaws are conspicuously broadened in large adults of _ferox_, and females of that population of _T. s. asper_ in the Atlantic Coast drainage. _Ratios_ Width of head increases at a rate slightly slower than does the length of the plastron (PL/HW, Fig. 3). The change in proportions is most pronounced at a plastral length of 7.5 to 8.0 centimeters. In general, the head is narrowest in _muticus_ and widest in _ferox_. _T. s. asper_ and _emoryi_ seemingly have the widest heads among the subspecies of _spinifer_. Geographically width of head increases from _spinifer_ and _hartwegi_ through _pallidus_ and _guadalupensis_ to _emoryi_. _T. ater_ terminates the cline; 12 specimens, ranging in plastral length from 9.6 to 18.4 centimeters, resemble _ferox_ and _asper_ in having wide heads (average PL/HW of 4.93). [Illustration: FIG. 3. Ratio of length of plastron to width of head (PL/HW) in some American species and subspecies of the genus _Trionyx_. The size of each sample is given in parentheses following an indication of the range (< = less than, > = greater than) in length of plastron (in cm.) of each sample. The horizontal line indicates the observed variation; the vertical line, the mean; the white rectangle, four standard deviations; and the black rectangle, four standard errors of the mean. There is some ontogenetic variation in PL/HW. The head is narrowest in _muticus_ and widest in _ferox_.] The carapace increases in width more slowly than it increases in length (CL/CW, Fig. 4). The change in proportions is most pronounced when the carapace is 8.0 to 8.5 centimeters in length. Ontogenetically _muticus_ varies least and _ferox_ most; large specimens of _ferox_ have narrower carapaces than _muticus_ of corresponding size. There is also an indication of a geographical gradient that parallels the cline mentioned above for PL/HW. There is a gradual decrease in width of carapace from _pallidus_ through _guadalupensis_ to _emoryi_. Of the subspecies of _spinifer_, _emoryi_ has the narrowest carapace and resembles _ferox_. In _T. ater_ this cline is accentuated and terminates; 12 specimens, ranging in plastral length from 9.6 to 18.4 centimeters, resemble _ferox_ and _emoryi_ in having narrow carapaces (average CL/CW of 1.32). _Osteological Characters_ Closure of the anterior, paravertebral fontanelles on the bony carapace, and size and number of plastral callosities are subject to ontogenetic variation (see sections entitled "Carapace" and "Plastron"). [Illustration: FIG. 4. Ratio of length of carapace to width of carapace (CL/CW) in some American species and subspecies of the genus _Trionyx_. Symbols as in Fig. 3. There is some ontogenetic variation in CL/CW (least in _muticus_). The carapace is narrowest in _ferox_ and _emoryi_, and widest in _muticus_, _pallidus_ and _asper_.]