and byssus, if calcified, are composed of aragonite. The outermost layer of the shell is the periostracum, a thin layer composed of horny conchiolin. The periostracum is secreted by the outer mantle and is easily abraded. The outer surface of the valves is often sculpted, with clams often having concentric striations, scallops having radial ribs and oysters a latticework of irregular markings.

In all molluscs, the mantle