In summary, _T. ferox_ occurs in all fresh-water habitats, but chiefly in lentic habitats in the northern part of its range where it and _T. s. asper_ are sympatric. _T. ferox_ possibly is more tolerant of brackish and marine waters than are the subspecies of _spinifer_ and _muticus_. The subspecies of _T. spinifer_ occur in all fresh-water habitats. In the southern part of the geographic range, which overlaps that of _T. ferox_, _T. s. asper_ occurs principally in running-water habitats. _T. s. pallidus_ and _T. s. asper_ are tolerant of brackish-water habitats. _T. s. guadalupensis_, known at present only from rivers and streams, occurs principally in river systems that drain the Edward's Plateau of southcentral Texas. _T. ater_ is confined to crystal-clear ponds in central Coahuila. The subspecies _muticus_ occurs in large rivers and streams throughout its geographic range, but is known from lakes and impoundments principally in the southern part of its range (the northernmost record is from Reelfoot Lake, Obion County, Tennessee); there is only one record of _muticus_ from a small, shallow, headwater creek (Reno County, Kansas), and only one from a lentic habitat (Meredosia Bay, Illinois) in the northern part of its range. _T. muticus calvatus_ is known at present only from rivers and streams. The seemingly greater restriction of _muticus_ to running-water habitats suggests less vagility than in _spinifer_ (Netting, 1944:86). Size and coloration are adaptations to habitat. Soft-shelled turtles of large size are best adapted to mesic, essentially continuous swampy or marshy habitats, whereas small size is an adaptation to less continuous, semi-isolated habitats. A turtle of the maximum size attained by _ferox_ in the habitat of _emoryi_ would, in a general way, probably be more conspicuous and exposed to its enemies, both in the aquatic environment and during overland excursions; perhaps the kind and amount of food would be insufficient. In any event, small size is correlated with the more arid habitats of the southwest, and large size with mesic ones in the southeast. _T. ferox_, the largest species, and the smallest population of _T. spinifer_ (resembling _muticus_) both occur in the southernmost part of the range of the genus. This situation does not support the corollary of Bergmann's Rule, that pertains to some groups of terrestrial reptiles, in which those subspecies occurring farther north, or in cooler climates during their season of activity, tend to be smaller. Within the species _spinifer_, the _emoryi_ group of subspecies are pallid having whitish dots on the carapace and lack extensive black pigmentation; these features seem to confer protective coloration on the inhabitants of arid, essentially sandy or muddy habitats having sluggish, turbid waters, whereas the more contrasting patterns of the _spinifer_ group of subspecies eastward seem more suited to existence in clearer, swifter waters.