Because _ferox_ resembles softshells from the Old World more closely than it does any American species, _ferox_ is assumed to be more closely related to Old World softshells than to any American species, and, because of resemblance to some fossils, _ferox_ is assumed to resemble most closely the primitive, ancestral stock of softshells that occupied North America. _T. spinifer_, _T. muticus_ and _T. ater_, which resemble each other more closely than any of them resembles _T. ferox_ or any Old World species, are considered autochthonous in North America. _T. spinifer_ and _T. muticus_ are distinct, sympatric species. Burt (1935:321) suggested that the two species "may be variants of the same species." _T. ater_ is weakly differentiated from _T. spinifer emoryi_. The species, _ferox_, _spinifer_ and _muticus_ are well-differentiated and were considered by Agassiz (1857), Gray (1869) and Baur (1893) as belonging to three different genera. In the widely distributed _T. spinifer_, the subspecies _spinifer_, _hartwegi_ and _asper_ closely resemble one another; _asper_ seems most distinct, whereas _spinifer_ and _hartwegi_ are terminal populations of an east-west cline in one character. The subspecies _pallidus_, _guadalupensis_ and _emoryi_ resemble one another more closely than any resembles any of the subspecies mentioned immediately above; _T. s. pallidus_, however, is annectent. _T. s. pallidus_ and _guadalupensis_ represent terminal populations of clines in several characters, some of which occur in _emoryi_, but that subspecies is more distinct from _pallidus_ and _guadalupensis_ than those subspecies are from each other. _T. s. emoryi_ is the most variable subspecies. _T. ater_, known only from a restricted area in central Coahuila, is most closely related to _T. s. emoryi_, and possesses some characters judged to represent the attenuation of the geographic cline in _pallidus_, _guadalupensis_ and _emoryi_ mentioned above. Some characters of _ater_ show alliance to the species _muticus_. Of _T. muticus_, whose geographic range is removed from that of _ater_, there are two subspecies. Four subspecies of _spinifer_ (_spinifer_, _hartwegi_, _pallidus_ and _asper_) intergrade in the Mississippi River drainage of Louisiana; few specimens, however, are typical of _asper_. The subspecies of _muticus_ do not show definite evidence of intergradation. To facilitate quick reference, the occurrence of some characters that are shared by, or are approximated in, two or more forms are listed in Table 10. In addition to external characters, some ratios emphasize the clinal relationship between _T. s. pallidus_, _guadalupensis_, and _emoryi_ mentioned above. Of especial interest is the frequent resemblance of those subspecies and _T. ater_ to _T. ferox_ (dorsal pattern on limbs of adults, reduction in anterior tuberculation, wide head, narrow carapace, and short snout), and the less marked resemblance of _T. muticus_ to _T. ferox_; not shown in