Perhaps Crassostrea virginica, the Eastern oyster? Volusia county, Florida; 08 June 2015.
Single specimen collected on sand flats.
C.nippona or C.gigas
Here's a new bivalve for me, a fluted, many-layered striped burgundy and white shell. It looks kind of like an oyster shell. These are two separate specimens, both found on rocks during the minus tide. One is under water.