Freshwater mussels are bivalved mollusks (Phylum Mollusca, Class Bivalvia) distantly related to marine clams, oysters, mussels, and scallops. They have soft inner bodies and hard outer shells consisting of two valves. Historically, North America had the richest fauna of freshwater mussels in the world. Biologists have discovered nearly 300 species of mussels living in the United States, most of which occupy streams and lakes east of the Rocky Mountains. Prehistoric American Indians ate the soft tissues and used the lustrous shells for making tools and ornaments. Historically, the shells have been prized as raw material for manufacturing buttons and cultured pearls.