What is (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 called?

operculum — (Zoöl.) (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. (Webster's 1913.)

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