Later, in the southwestern part of the range of the MSA stock (southern Texas and northern México), the SA and _muticus_ stocks were separated. The _muticus_ stock occurred to the northeastward, and presumably no farther south than the area included within the present drainage basin of the Colorado River. Southward, the SA stock was isolated into several populations that are today represented by _ater_ and _T. s. emoryi_, the most variable subspecies; the distribution of the most distinctive population of _emoryi_ indicates a former isolated inland drainage. The multiple fragmentation of the SA stock presumably terminated by the end of the Pliocene. The progenitors of _T. ater_ probably closely resembled _ferox_. _Trionyx ater_ and _T. ferox_ resemble each other morphologically and in habitat. Therefore, the progenitors of _ater_ are considered to have undergone comparatively little differentiation. The _spinifer_ stock, occurring principally in the area included within the present drainage basin of the Río Grande, extended its geographic range eastward and became sympatric with _muticus_ and _ferox_. An expansion of range necessarily demands more mesic conditions; these were perhaps afforded by the pluvials (wet, rainy ages) that were coincident with the glacial periods in the Pleistocene (Antevs, 1948:168). The pluvials permitted the isolated populations of the _spinifer_ stock to unite, and permitted that stock to extend its range eastward. The concurrent continental glaciation permitted the _spinifer_ stock to extend its range eastward only in a belt approximately 300 miles wide along the Gulf Coast, and also displaced the ranges of _ferox_ and _muticus_ to southern latitudes. Perhaps _ferox_ was less tolerant of decreased temperatures or changes in habitat than was the _spinifer_ stock but, for some unknown reason, _ferox_ did not extend its range westward. Because _T. ater_ closely resembles _T. s. emoryi_, continued isolation of _ater_ since the beginning of the Pleistocene seems unlikely and _ater_ may have been reunited in subsequent pluvial periods with the _spinifer_ (_emoryi_) stock. A climatic fluctuation between relatively wet and dry periods is corroborated by studies of soil profiles in Trans-Pecos Texas (Bryan and Albritton, 1943).