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 [451] 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").
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.
