 The shell is composed of two calcareous valves held together by a ligament . The valves are made of either calcite , as is the case in oysters , or both calcite and aragonite . Sometimes , the aragonite forms an inner , nacreous layer , as is the case in the order <unk> . In other taxa , alternate layers of calcite and aragonite are laid down . The ligament and byssus , if calcified , are composed of aragonite . The outermost layer of the shell is the periostracum , a skin @-@ like layer which is composed of a horny organic substance . The periostracum is secreted in the groove between the outer and middle layers of the mantle , and is usually olive or brown in colour and 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 . 
