The separation of the range of _spinifer_ in the general region of western Louisiana, resulting in the differentiation of the _spinifer_ group of subspecies to the east and the _emoryi_ group of subspecies to the west, and the differentiation of _T. s. asper_ and _T. m. calvatus_, both having corresponding western limits of distribution (Mississippi River drainage), are associated with the activities of the Mississippi River and its flood-plain. The combined effects of the pluvials and interpluvials seem responsible for changes in the lower Mississippi Valley. Great volumes of summer melt-water in the glacial stages greatly increased the breadth of the channel of the lower Mississippi River (corresponding to the northern extent of the Mississippi embayment; Hobbs, 1950), and this, coupled with the encroachment of Pleistocene seas (especially in the Mississippi embayment) in the interglacial periods, perhaps separated populations eastward represented today by _T. m. calvatus_ and _T. s. asper_. The _spinifer-hartwegi_ stock probably developed in southern Louisiana in association with the meandering of the Mississippi River and its tributaries, and its broad alluvial plain. The biota of that plain differed from that adjacent to the east or west (see discussion in Viosca, 1944) and constituted a barrier, of a sort, to free communication between the east and west. Westward the _emoryi_ group of subspecies differentiated, its eastern limit probably being the Red River, which followed its own course to the Gulf along the lowlands on the west side of the Mississippi Valley and did not empty directly into the Mississippi until Recent times (Holland, 1944:20). There was not an equally-marked, corresponding separation of the range of _muticus_. However, the juvenal pattern of the subspecies _muticus_ that inhabits the Gulf Coast streams is slightly different (having less short lines) from that of _muticus_ elsewhere. The Río Grande (inhabited by _emoryi_) presumably had its own exit to the Gulf whereas rivers westward to (and including) the Red River (inhabited by _pallidus-guadalupensis_ cline) probably were joined near their mouths forming a large drainage system. Hubbs (1957:93) pointed out that the Río Grande-Nueces divide also limits a large number of species of fish. The differentiation of _pallidus_ and _guadalupensis_ is possibly due to a difference in the salt content of waters that drain the Edward's Plateau (see page 547), or to isolation of those subspecies in separate drainage systems that had their own exits to the Gulf. In the lower Mississippi drainage, the _spinifer-hartwegi_ stock extended its range northward following the retreat of the last glacial stage, and differentiated into those two subspecies in the upper Mississippi drainage and Great Lakes-St. Lawrence drainage system.