Periostracum's a thin organic coating or "skin" which's the outermost layer of the shell of many shelled 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's an integral part of the shell, and it forms as the shell forms, along with the other shell layers. Periostracum's visible as the outer layer of the shell of many molluscan species from terrestrial, freshwater and marine habitats,… (More on Periostracum)