\O*per"cu*lum\, n.; pl. L. {Opercula}, E.
{Operculums}. [L., a cover or lid, fr. operire to cover.]
1. (Bot.)
   (a) The lid of a pitcherform leaf.
   (b) The lid of the urnlike capsule of mosses.
2. (Anat.)
   (a) Any lidlike or operculiform process or part; as, the
       opercula of a dental follicle.
   (b) The fold of integument, usually supported by bony
       plates, which protects the gills of most fishes and
       some amphibians; the gill cover; the gill lid.
   (c) The principal opercular bone in the upper and
       posterior part of the gill cover.
3. (Zo["o]l.)
   (a) The lid closing the aperture of various species of
       shells, as the common whelk. See Illust. of
       {Gastropoda}.
   (b) Any lid-shaped structure closing the aperture of a
       tube or shell.