Males of the subspecies of spinifer develop small, sharp tubercles on the dorsal surface of the carapace when sexually mature. As growth proceeds, the minute prominences along the anterior edge of the carapace on hatchlings of both sexes of spinifer change in shape to conical projections or low, flattened, scarcely-elevated prominences, depending on the subspecies ([Fig. 8]).
Large females of spinifer and ferox acquire enlarged, flattened knobs in the nuchal region and posteriorly in the center of the carapace.
Length of Tail
The preanal region of the tail rapidly elongates in males of all soft-shells when they are sexually mature.
Width of Alveolar Surfaces of Jaws
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 [450] 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).
