Clam is a common name for several species of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams have two shells of equal size connected by two adductor muscles and have a powerful burrowing foot. They live in both freshwater and marine environments; in salt water they prefer to burrow down into the mud and the turbidity of the water required varies with species and location; the greatest diversity of these is in North America. Clams in the culinary sense do not live attached to a substrate (whereas oysters and mussels do) and do not live near the bottom (whereas scallops do). In culinary usage, clams are commonly eaten marine bivalves, as in clam digging and the resulting soup, clam chowder. Many edible clams, such as palourde clams, are ovoid or triangular; however, razor clams have an elongated parallel-sided shell, suggesting an old-fashioned straight razor. Some clams have life cycles of only one year, whilst at least one reached an age of more than 500 years. All clams have two calcareous shells or valves joined near a hinge with a flexible ligament and all are filter feeders.