The clam is a bi-valve mollusk of the Pelecypoda class that digs in the sand. Although native to both salt and fresh water, saltwater clams are considered far superior for eating purposes. The word clam comes from the Old English clamm, meaning "bond" or "fetter," referring to its tightly clamped shell. Native Americans carved clam shells into beads and used them as currency or wampum (Algonquian meaning "white string of beads"), and introduced colonists to the concept of clambakes. The National Marine Fisheries located in Milford, Connecticut, pioneered clam farming circa 1930. Commercial hatcheries gained their foothold in the Northeast in the 1960s. Most commercially-available clams are nowadays raised on farms. Since the shells are built of calcium deposits, it is no wonder that clams are a good source of calcium as well as being high in protein.