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 molluscs it is primarily seen in snails and clams, i.e. in gastropods and bivalves, 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.

Periostracum is visible as the outer layer of the shell of many molluscan species from terrestrial, freshwater and marine habitats, and may be seen in land snails, river mussels and other kinds of freshwater bivalves, as well as in many kinds of marine shelled molluscs.

The word "periostracum" means "around the shell", meaning that the periostracum is wrapped around what is usually the more calcareous part of the shell. Technically the calcareous part of the shell can (at least in theory) be referred to as the "ostracum", but that term is only very rarely used.

This shell layer is composed of a type of protein known as conchiolin. Conchiolin is largely composed of quinone-tanned proteins, which are similar to those found in the epidermal cuticle.

The formation of a shell requires certain biological machinery. The shell is deposited within a small compartment, the extrapallial space, which is sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs. This caps of the extrapallial space, which is bounded on its other surfaces by the existing shell and the mantle. The periostracum acts as a framework from which the outer layer of carbonate can be suspended, but also, in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur. The accumulation of ions is driven by ion pumps packed within the calcifying epithelium. The organic matrix forms the scaffold that directs crystallization, and the deposition and rate of crystals is also controlled by hormones produced by the mollusc. The periostracum was probably essential in allowing early molluscs to obtain large size with a single valve.

...
Wikipedia