Evidence of the relative age of two genera of turtles, as suggested by their degree of differentiation into minor taxa, and the degree of difference between populations of two genera that inhabit adjacent drainage systems, may indicate the relative ages of particular river systems. For example, the slight resemblance of _G. versa_ to _kohni_ and the close resemblance of _T. s. guadalupensis_ to _pallidus_ in Texas may reflect the age of the genus _Trionyx_ and the youth of the genus _Graptemys_. Remembering that the genus _Graptemys_ is relatively recently evolved and assuming _G. versa_ to be the most primitive and ancestral species of the genus (at least it is monotypic, the most aberrant species, and unlike any other species of the genus), it seems logical to suppose that the physiographic changes responsible for the Colorado-Brazos divide and the isolation of _versa_ occurred early in the evolutionary history of the genus _Graptemys_. The degree of differentiation of _Trionyx_ suggests that that genus is, comparatively, much older, and that the same physiographic changes responsible for the Colorado-Brazos divide and differentiation of the subspecies _pallidus_ and _guadalupensis_ occurred late in the evolutionary history of the genus _Trionyx_. In general, patterns of distribution of turtle populations support physiographic evidence concerning changes in stream confluence and relative age of river systems. SUMMARY In North America, soft-shelled turtles (genus _Trionyx_) occur in northern México, the eastern two-thirds of the United States, and extreme southeastern Canada. The genus fits the well-known Sino-American distributional pattern. In North America there are four species. Three (_ferox_, _spinifer_ and _muticus_) are well-differentiated and one (_ater_) is not well-differentiated from _spinifer_. Characters of taxonomic worth are provided by the following: size; proportions of snout, head and shell; pattern on carapace, snout, side of head, and limbs; tuberculation; sizes of parts of skull; number of parts of carapaces; and, shape and number of some parts of plastra. Many features show geographical gradients or clines. _T. ferox_ is the largest species and _muticus_ is the smallest. Females of all species are larger than males. With increasing size of individual, the juvenal pattern is replaced by a mottled and blotched pattern in females of all species; adult males of _spinifer_ retain a conspicuous juvenal pattern, whereas the juvenal pattern is sometimes obscured or lost on those of _ferox_ and _muticus_. The elongation of the preanal region in all males, and the acquisition of a "sandpapery" carapace in males of _spinifer_ occur at sexual maturity. There is a marked secondary sexual difference in coloration in a population of _T. s. emoryi_ (side of head bright orange in males and yellow in females). The sex of many hatchlings of _T. s. asper_ can be distinguished by the pattern on the carapace. Slight