Perhaps Crassostrea virginica, the Eastern oyster? Volusia county, Florida; 08 June 2015.
Single specimen collected on sand flats.
Crassostrea virginica - Eastern Oyster
Chincoteague NWR, VA April 4, 2015
Shells on the beach
The eastern oyster (Crassostrea virginica) — also called Atlantic oyster or Virginia oyster — is a species of true oyster native to the eastern seaboard and Gulf of Mexico coast of North America. It is also farmed in Puget Sound, Washington, where it is known as the Totten Inlet Virginica. Eastern oysters are and have been very popular commercially. Today, less than 1% of the original 17th century population (when the original colonists arrived) is thought...