Periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it's primarily seen in snails and clams, for example in bivalves and gastropods, but it's also found in cephalopods such as the nautilus. Periostracum is an integral part of the shell, and it forms as the shell forms, along with the other shell layers. Periostracum is visible as the outer layer of the shell of many molluscan spec… (More on Periostracum)