Large females of _T. spinifer_ have obtuse prominences in the center of the carapace posteriorly, some of which in many specimens are arranged in longitudinal rows; I cannot discern any correlation of number or arrangement of prominences with size in _spinifer_ or _ferox_. The carapace in adult males of _spinifer_ bears small, sharp tubercles that make the surface feel like sandpaper. The tubercles on the anterior edge of the carapace in adults of both sexes vary from round to equilateral and conical to low and flattened (see comments on tuberculation under subsection entitled "Geographic Variation"). Some large females of the same subspecies have tubercles on the anterior edge of the carapace that may be conical (higher than wide) or equilateral. The difference in shape of the tubercles seems not to be correlated with size because one _T. s. pallidus_, 30.5 centimeters (TU 13212) has prominent but blunted and equilateral tubercles, whereas, another female of _pallidus_, 20.8 centimeters (TU 13210), from the same locality has higher, conical tubercles. The blunted, equilateral tubercles may be the result of environmental wear, or the difference in shape of tubercles may be due to individual variation. _Pattern on Carapace_ Two features of the pattern on the carapace are of taxonomic worth: 1) the width and distinctness of the pale rim at the periphery of the carapace (marginal rim), if present, and 2) the kind of pattern on the carapace (juvenal pattern). The marginal rim is absent in females of _T. ater_, and only faintly evident in males. The marginal rim is obscured or absent (adult males and females) and is not separated from the ground color of the carapace by a dark marginal line in hatchlings of _T. ferox_. The carapace of _T. muticus_ has a marginal rim that is usually separated from the ground color of the carapace by an ill-defined, dark marginal line; some individuals lack the marginal dark line. The subspecies of _T. spinifer_ have a well-defined, dark, marginal line that separates the marginal rim from the ground color of the carapace; _T. s. asper_ has more than one dark marginal line on the carapace. The marginal rim is ill-defined and blotched, or absent, in large females of all species of _Trionyx_. The marginal rim is widest at the posterior end of the carapace and lacking in the nuchal area. The width of the pale marginal rim is very narrow, almost to the degree of being absent, in juveniles of _T. ferox_. _T. s. emoryi_ has a pale, marginal rim that is four or five times wider posteriorly than it is laterally, whereas posteriorly the width of the rim in the other subspecies of _T. spinifer_ and in the species _T. muticus_ is only two or three times wider posteriorly than it is laterally.