The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals.   

The word "periostracum" means "around the bone".  The periostracum wraps around the more calcareous part of the shell.

Periostracum is visible as the outer layer of the shell of many species of clams.


The periostracum is often yellowish or brownish in color and appears as a varnish-like substance. 

This is seen in the image on the left (Northern Quahog, Mercenaria mercenaria).

In some cases it is black and rough with sections wearing off as illustrated in the image on the right (Ocean Quahog, Arctica islandica).