the lid of a pitcherform leaf.-the lid of the urnlike capsule of mosses.-any lidlike or operculiform process or part; as, the opercula of a dental follicle.-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.-the principal opercular bone in the upper and posterior part of the gill cover.-the lid closing the aperture of various species of shells, as the common whelk. see illust. of gastropoda.-any lid-shaped structure closing the aperture of a tube or shell.