Elliptio is a genus of medium- to large-sized freshwater mussels, aquatic bivalve mollusks in the family Unionidae, commonly known as the unionids, freshwater mussels or naiads. In contrast with many other groups of American Unionidae, the Elliptio species reach their greatest diversity in the Atlantic-draining rivers of Georgia and the Carolinas, and large parts of Florida. One species ranges north into New England and southern Canada, and two occur in the interior Mississippi drainages.