Periostracum

The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as Allonautilus scrobiculatus. Periostracum is an integral part of the shell, and it forms as the shell forms, along with the other shell layers.