Pelecypods (peh-les'-i-pods) include oysters, clams, mussels, and cockles. They have been found in some of the oldest marine rocks known and still are very numerous in the seas and rivers today. In the past, some pearl buttons were made from clam shells from the Illinois and Mississippi Rivers. This major industry caused near extinction of some species.
Most pelecypods, also know as bivalves, have two shells that are mirror images of each other, one on the right and one on the left. Each shell has a beak that points forward and represents the spot where the shell began to grow. The top edge of each shell commonly has several teeth and sockets that fit into those of the opposite shell to make a hinge. The outside of the shell generally is ornamented by ribs, spines, and growth lines.
Most pelecypods form shell banks in the seas or rivers on sand and mud flats. Many burrow into the mud or sand and even into wood or rock. Some oysters attach themselves to rocks, and others creep about the sea floor by means of a hatchet-shaped foot thrust between the open valves. A few (scallops) move by jet propulsion, forcing water in a jet stream from openings at the beak end.
Fossil clams are common in some Pennsylvanian rock formations in central Illinois and in some Ordovician limestones in northern and western Illinois.