ontogenetic variation occurs in some proportional measurements. Large skulls of _ferox_ and some _asper_ (those in Atlantic Coast drainages) have expanded crushing surfaces on the jaws. Considering osteological characters, _muticus_ is most distinct; there is less difference between _ferox_ and _spinifer_ than between those species and _muticus_. _T. ferox_ is monotypic, confined to the southeastern United States, and resembles Old World softshells more than it does any American species. The northern part of the geographic range of _ferox_ overlaps that of _T. s. asper_; there, the two species are ecologically isolated. _T. spinifer_ is polytypic, has the largest geographic range, and is composed of six subspecies, of which two are described as new (_pallidus_ and _guadalupensis_). The subspecies are divisible into two groups. One, the _spinifer_ group (_spinifer_, _hartwegi_ and _asper_) is recognized by a juvenal pattern having black spots or ocelli; _asper_ is the most distinctive and shows little evidence of intergradation in the lower Mississippi River drainage with the _spinifer-hartwegi_ complex, which, northward, is differentiated into two subspecies in which there is an east-west cline in size of the ocelli on the carapace. The _emoryi_ group (_pallidus_, _guadalupensis_, _emoryi_) is recognized by a pattern of white spots; _emoryi_ is most distinctive. Each of several characters behaves as a cline if traced from east to west through the three subspecies. _T. s. pallidus_ intergrades with the _spinifer-hartwegi_ complex in the lower Mississippi River drainage. _T. s. emoryi_ is the most variable subspecies; in its most notable population the males have orange coloration. _T. s. emoryi_ has been introduced into the Colorado River drainage of Arizona. _T. ater_ most closely resembles _T. s. emoryi_, but shows alliance with _T. muticus_ and _T. ferox_. _T. ater_ is confined to ponds of crystal-clear water in central Coahuila, México. _T. muticus_ is completely sympatric with _spinifer_, and is composed of two subspecies (_muticus_ and _calvatus_). _T. m. calvatus_ shows no evidence of intergradation in the lower Mississippi River drainage with _T. m. muticus_, corresponding somewhat to the relationship of _T. s. asper_ with the intergradient population of _T. spinifer_ in the Mississippi River.