Corneous

Corneous is a biological and medical term meaning horny, in other words made out of a substance similar to that of horns and hooves in some mammals.

The word is generally used to describe natural or pathological anatomical structures made out of a hard layer of protein.

The word corneous is also often used to describe the operculum of a snail, a gastropod mollusc.

Not all gastropods have opercula, but in the great majority of those that do have one, the operculum is corneous.

(However in several genera within a few families including the marine Naticidae and the terrestrial Pomatiidae, the operculum is primarily calcareous, in other words mostly made of calcium carbonate.)