and exclusive in neogastropods.

There are two basic types of opercula in terms of their material composition:
* The most common kind of operculum is composed of a thin to rather thick corneous protein material, which is yellow to brownish in color and is usually somewhat translucent. This matter is supple when in its natural state but may become brittle when it is dried out. The operculum varies in shape, depending on the family of snails and the shape of the aperture of their shells.
* The other kind of operculum is restricted