Three major faunal provinces have been defined for the east coast of the United States, based upon a Q-mode analysis of 110 living bivalves and 70 gastropods. The major discontinuities in distribution exist at Cape Cod and at Cape Hatteras. The Hatteras break can be attributed to a temperature barrier imposed by the Gulf Stream, while the Cape Cod break can be attributed to a temperature barrier imposed by southward-flowing arctic currents. The distribution of Miocene mollusks, when analyzed by cluster analysis and compared to the recent model, shows no major discontinuity between Florida and New Jersey. Also, no discontinuity is observed when only the percent of species crossing station boundaries is considered. It thus appears that within this region, no major temperature barrier to molluscan distribution existed during the Miocene.