Pelecypods have two shells, or bivalves, that protect the soft parts of the animal. The valves are generally of equal size (except in groups like the oysters) and shape and are hinged at the back. Some types, such as oysters, live in large groups that create beds or low-relief banks of shells, where the animals feed by filtering plankton and organic debris from the water. Other bivalves burrow through the mud or swim about in search of debris to eat. Many bivalve fossils in Delaware are preserved as steinkerns. Pelecypods are abundant in the spoils from the Mount Laurel Formation on both sides of the Canal in the vicinity of Reedy Point.