Oyster shells are the hard outer coverings of oysters, marine bivalves whose two valves meet at a hinge. The shell protects the animal's soft body and grows in layers, often developing a thick, uneven surface with ridges and irregular edges. Oysters typically attach to hard surfaces in coastal waters, and the shape of a shell can reflect its position and the conditions around it as it grows. Unlike the smooth, rounded shells people may associate with other mollusks, oyster shells are commonly rough and asymmetrical, with one valve more deeply cupped than the other. These features make the oyster shell distinctive among bivalve shells.