 Some bivalves , including the true oysters , the jewel boxes , the jingle shells , the thorny oysters and the kitten 's paws , cement themselves to stones , rock or larger dead shells . In oysters the lower valve may be almost flat while the upper valve develops layer upon layer of thin horny material reinforced with calcium carbonate . Oysters sometimes occur in dense beds in the neritic zone and , like most bivalves , are filter feeders . 
