The Lower Tertiary Gulf Coast was subjected to repeated transgressive-regressive cycles of varying rate and extent which had a marked effect on species longevities. In some neogastropods dispersal and geographic distribution played important roles in species longevities. Species with planktic larval stages that inhabited broad areas of the Gulf Coast could maintain their distributions even during regressive phases when delta building was strong. Wide distributions, free gene flow, and resistance to isolation gave planktic species an evolutionary stability that resulted in greater species longevity. Species with nonplanktic development could attain wide geographic ranges when barriers were minimized, but they were unable to maintain their distributions in the face of regressive progradation. Thus their populations were isolated, and extinction and speciation accelerated, giving them lesser evolutionary longevity. Although planktic volutids tend to be widespread, planktic fasciolariids and mitrids were often restricted in distribution and therefore probably stenotopic. If dispersal and environmental tolerance are considered as separate forces, they seem to act in combination to produce very long, very short or intermediate species longevities.