Foraminiferal communities in the Cenozoic shelf deposits of the North American Atlantic Coastal Plain exhibit little unity during almost 55 million years of successive transgressions and regressions. Transgression communities are composed of a dynamic mixture of immigrants and newly evolved species. During regressions, species within these communities either became extinct or emigrated. Some emigrants returned during subsequent transgressions, but many did not. The neritic species of the Atlantic and Gulf continental margins constitute a species pool. Immigrants and emigrants transferred into and out of the species pool, while extinctions and originations repeatedly altered its species composition. While the results indicate a lack of local community unity, at the same time they demonstrate the necessity of a species pool to sustain species diversity.