Images: 

Do you have an opinion as to what this might be?  It looked like a large (+/- six inches) clam to me.  The top valve was attached to the bottom of an overhang.  I took the pictures on July 5, 2007, on the wreck of the Benwood, buoy BW5, Key Largo, Florida, depth 35 feet, bottom temperature 84 degrees Fahrenheit.  An informed source suggests, “A “rock jingle” or “rock oyster.” ...  Perhaps in the genus Pododesmus.”