Bivalves are aquatic mollusks that possess two valves that protect the soft body parts. The valves are secreted by the mantle, a soft tissue that leaves a scar (the pallial line) where it connected to the inside of the shell. The muscles that hold the two valves together also leave scars on the inside of the shell, as do the siphons, through which food passes in and out of the body. The length and shape of the siphon scar, or pallial sinus, is helpful for interpreting the burrowing habits of a bivalve. Different scars are also characteristic of different bivalve species.
Unlike the valves of brachiopods, clam valves are typically of similar size and shape. The two shells are hinged together by a strong ligament. Small teeth and sockets may also be present in the hinge area, which add strength to the hinge and prevent the two valves from slipping apart.
Bivalves are filter feeders: They strain small food particles from water. Water enters through the inhalant siphon, passes through the gill, which takes in oxygen from the water and exits through the exhalent, the outcurrent siphon. The gill traps food particles and transports them to the mouth. Bivalves live in a variety of habitats: Some live epifaunally (on top of seafloor sediment); others live infaunally (in seafloor sediment), using their muscular foot to burrow within the bottom sediments. The shape of the shell can be related to the environment in which the animal lives.
Bivalves originated in the Cambrian period but did not become abundant until the Ordovician period. They are not particularly common in the Paleozoic rock of Wisconsin, but they sometimes can be found within large groupings of other Ordovician fossils.