Twelve freshwater gastropod families, 39 genera and 70 fossil and Recent species are recognized and systematically treated. Keys to the families, genera and species were constructed and all species are illustrated. The purpose of this study was to synthesize and update the taxonomy, zoogeography, origins and correlations with drainage systems of the entire Recent and fossil freshwater gastropod fauna of Texas.

The holomorphology of ll life stages of 20 western Nearctic Isoperla and one Cascadoperla was studied over the 3-year period 1975-1978. One monotypicgenus new to science, Cascadoperla, is described, and Cascadoperla tricture (Hoppe) designated as the type species. The nymph, adult male and female, and ova are described and illustrated.