shell money, medium of exchange consisting of shells, the most widely distributed type of ancient currency. Shells are particularly useful as money because they may be strung in long strips of proportionate value or they may be used to provide a single unit value in exchange. Shells ultimately derived their value from their use as jewelry and in rituals. Relative scarcity of the type of shell used or the way the shell is fashioned often determines its value. Cowrie shells have been the most common shell media and are probably the oldest in usage for exchange. Wampum, used in North America, was usually fashioned from thick-shelled clams; dentalia, or tooth shells, were popular with the coastal Native Americans of W North America. Mother-of-pearl and tortoiseshell are said to have been used for trade in ancient China. Oceanic peoples in particular use a variety of shells in trade.