_Natural history._--Specimens of _P. s. taylori_ were caught in hoop nets in clear deep pools and in the Río Chiquito. No specimens were collected or observed in marshy situations where the water was shallow or stagnant. Individuals were seen only near dusk and in early morning when a number floated just below the surface with only their heads showing. They were never seen on land during our short stay in the basin. The few stomachs that were opened contained vegetable material. In terms of number of specimens trapped, _P. s. taylori_ was the most abundant turtle in pools at and near the type locality (Webb and Legler, 1960). _Relationships and phylogeny._--The basin of Cuatro Ciénegas now drains, via the Río Salado, into the lower Río Grande. Brief descriptions of habitats and topography in the basin are given by Gilmore (1947:148-150, fig. 2) and Webb and Legler (1960). In the more northern parts of the Salado drainage (for example, in the Río Sabinas near Múzquiz) slider turtles are typical _P. s. elegans_. Assuming that conditions which permit genetic exchange between populations of turtles in the Salado drainage system differ in no major respect from conditions in other parts of the range of _Pseudemys scripta_, it is logical to suppose that the differentiation of _P. s. taylori_ at Cuatro Ciénegas was preceded by the isolation of a population in that basin. The Río Chiquito drains through a narrow gap in the northeastern end of the basin of Cuatro Ciénegas. Interruption of this stream would effectively isolate aquatic habitats in the basin. It is here proposed that _P. s. taylori_ is a relict of an earlier, lower Río Grande stock, part of which became isolated in the basin of Cuatro Ciénegas in postpluvial times. The morphological similarity of _P. s. taylori_ and _P. s. elegans_ indicates that both were derived from this parent stock; similarity of both subspecies to populations of _P. s. ornata_ in Tamaulipas suggests that the latter subspecies may also be a derivative of the mentioned stock of the lower Río Grande.