Periostracum

Periostracum on a shell.

Periostracum is a thin coat or skin on the outside of the shells of molluscs, brachiopods and other animals with shells.^[1]

Periostracum Media

The shell of Stenotrema florida, a land snail or terrestrial gastropod. The periostracum of this species has minute hairs, giving the snail a velvety feel
A yellowish tan periostracum is visible on the lower two thirds of this juvenile (8 cm) valve of the marine bivalve Spisula solidissima
The dark periostracum is flaking off of this dried-out valve of the "ocean quahog", marine bivalve Arctica islandica, from Wales
Fresher valves of Arctica islandica with periostracum intact. Note that the periostracum is partially worn off on the umbo, this is because the umbo is older than the rest of the shell and also projects more: it has been exposed to more abrasion during the life of the clam
On this Conus leopardus, natural rubbing has eroded the periostracum in some parts of the shell.
Distorsio ventricosa, an example of "hairy" gastropod.