O-pĕr'eu-lŭm, n.

n. [Lat., a cover or lid, from operire, to cover.]

1. (Bot.) (a.) The lid of a pitcher-form leaf. (b.) The lid of the urn-like flower capsule of mosses.
2. (Conch.) (a.) The lid closing the aperture of various species of snails' shells, as the common whelk. (b.) Any lid-shaped structure closing the aperture of a tube or shell.
3. (Ichth.) The apparatus, supported by bones, which protects the gills of fishes.