Periostracum

thumb|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]] thumb|A yellowish tan periostracum is visible on the lower two thirds of this juvenile (8 cm) valve of the marine bivalve Spisula solidissima thumb|The dark periostracum is flaking off of this dried-out Valve (mollusc)|valve of the "ocean quahog", marine [[bivalve Arctica islandica, from Wales]] thumb|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 thumb|On this Conus leopardus, natural rubbing has eroded the periostracum in some parts of the shell. thumb|Distorsio ventricosa, an example of "hairy" gastropod.

The periostracum ( ) is a thin, organic coating (or "skin") that 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. The periostracum is an integral part of the shell, and it forms as the shell forms, along with the other shell layers. The periostracum is used to protect the organism from corrosion.