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From The Collaborative International Dictionary of English v.0.48:
Operculum \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. [1913 Webster] 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. [1913 Webster] 3. (Zool.) (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. [1913 Webster]