This is a very good find! This is a Fossilized Mercenaria species Clam from Florida.   The specimen has the partially complete large clam shell with attached matrix.  The matrix is composed of calcited cemented marine fossils. This large clam fossil specimen including the matrix measures 5 3/4″ x  4″ x 1 1/4″ thick.  This is a very nice representative specimen from Rucks Pit, Okeechobee County, Fort Drum, Florida.