A comparison of the distributions of _Trionyx_ and _Graptemys_ in Texas suggests a faunal break between the drainage systems of the Brazos and Colorado rivers. _Graptemys versa_ occurs in the Colorado and Guadalupe-San Antonio drainages. To my knowledge _versa_ hitherto has not been recorded from the latter drainage system. I have seen one specimen of _Graptemys_ (custody of Gerald Raun, University of Texas) from the Guadalupe River drainage, which I judge to be representative of _versa_, and Olson (1959:48) has reported _Graptemys_ (probably _versa_) in the San Antonio River. The distribution of _G. versa_ parallels in a general way, the distribution of _T. s. guadalupensis_. _G. kohni_ and _T. s. pallidus_ occur in the Brazos River and eastward. Also, it is notable that the population of _T. m. muticus_ occurring in the Colorado River drainage differs slightly (more black pigmentation) from the same subspecies in the adjacent Brazos River system. There is much difference in the patterns of distribution and degree of differentiation of different genera of aquatic turtles in the eastern United States. Tinkle (1958:41-43, Figs. 49-55) concluded that a general resemblance in the patterns of distribution of the different genera of turtles was evidence that the rates of evolution were essentially the same, assuming that each genus had had a similar time interval for differentiation (_op. cit._:42). If this is true, corresponding patterns of distribution might indicate the same relative age of the population of turtles concerned. Generally, the genera of turtles that on morphological grounds are considered the oldest and most primitive (_Macroclemys_, _Chelydra_) show less differentiation into species and subspecies than those considered younger and more recently evolved (_Graptemys_, _Pseudemys_). In the genus _Graptemys_, much differentiation occurs in the geologically, recently formed, Gulf Coast drainage systems of the southeastern United States. It would seem then, that faster rates of differentiation denote more recent genera, whereas older genera are endowed with a "genetic senility" and are less subject to change.