I have seen one specimen (UMMZ 59198) from the eastern part of the Tennessee drainage (inhabited by _T. s. spinifer_) that resembles _T. s. asper_ (occupying the Gulf Coast drainages of the southeast). This resemblance tends to support the thesis of a former confluence of the Coosa (Alabama River system) and Tennessee drainages as believed by some malacologists to explain resemblances in molluscan fauna and as corroborated by physiographical evidence (see discussion in van der Schalie, 1945). The Importance of the Study of Turtle Populations in Relation to the History of River Systems In the Río Grande drainage the geographic distribution of the population of _emoryi_ having orange color in males is approximately the same as that of _Pseudemys scripta gaigeae_; the corresponding distributions suggest that a part of the Río Grande drainage consisting of the Río Conchos in Chihuahua and the Big Bend region of Texas was isolated in former times. Accordingly, the known aquatic chelonian fauna in the basin of Cuatro Ciénegas in central Coahuila, México, is endemic (except _T. s. emoryi_). And the coincidence of the geographic ranges of _T. muticus calvatus_ and _Graptemys pulchra_ in the southeast suggest a former association of the included (Pearl to Escambia) river systems. The occurrence of _T. s. pallidus_ in the Red River drainage indicates that the Red River was formerly associated with the Gulf Coast streams of eastern Texas and western Louisiana (inhabited by _pallidus_) and not with the Mississippi River drainage. The lower Mississippi River valley forms a prominent barrier to the eastern and western dispersal of many kinds of species and subspecies of turtles. _T. m. calvatus_ and _T. s. asper_, which occur in rivers of the Gulf Coast drainage east of the Mississippi, are well-differentiated subspecies showing little or no evidence of intergradation with their relatives in the Mississippi River. The large faunal break provided by the Mississippi River would seem to indicate greater age for that river than for other rivers of the Gulf Coast drainage.