The following ratios were developed from the measurements. Reference to these ratios will be made by the abbreviations within the parentheses: length of carapace/length of plastron (CL/PL); length of carapace/width of carapace (CL/CW); length of carapace/plane of width of carapace (CL/PCW); length of plastron/width of head (PL/HW); width of head/length of snout (HW/SL); diameter of ocellus/length of plastron (OD/PL). Secondary Sexual Variation _Size_ In many species of turtles, females are larger than males; the difference in size between the sexes is probably most pronounced in aquatic emydids. The ten largest individuals of each sex were selected to indicate the relative difference in size between the sexes of the three American species of _Trionyx_ (excluding _ater_, Table 2). Female soft-shelled turtles attain a larger size than males. _T. ferox_ is the largest species; _muticus_ is the smallest. The approximate maximal size of each sex and the difference in size between the sexes are more correctly expressed for _spinifer_ and _muticus_ than for _ferox_, because fewer specimens of _ferox_ were examined; presumably the approximate maximal size of males and females of _ferox_ is larger than is indicated in Table 2. TABLE 2. Secondary Sexual Difference in Maximal Size of North American Species of the Genus Trionyx (excluding ater) Based on the Ten Largest Specimens of Each Sex of Each Species. The Extremes Precede the Mean (in parentheses). =============+============================ SPECIES | Plastral length (cm.) -------------+---------+------------------ _ferox_ | males | 17.0-26.0 (20.0) | females | 23.3-34.0 (27.9) | | _spinifer_ | males | 13.8-16.0 (14.4) | females | 26.0-31.0 (28.0) | | _muticus_ | males | 11.8-14.0 (12.3) | females | 17.7-21.5 (18.9) -------------+---------+------------------ _Pattern_ Secondary sexual differences in pattern are probably more pronounced in soft-shelled turtles than in other species of turtles, except perhaps for the well-known melanism and concomitant obliteration of pattern acquired by some adult males of the _scripta_ section of the genus _Pseudemys_.