the operculum is the plate closing off the shell aperture when the animal is retracted; it consists of whorls around a nucleus or gyre; for the types A, B and C, sometimes distinguished- see illustration below.
In Atlanta sometimes a size formula (A : B : C) is used to express the shape of the operculum.

Alternative forms for operculum : opercula, opercular, opercule, operculum formula.