plica

plica (noun; plural -cae; Latin plicare, to fold): 1. A bend, fold, wrinkles, crenulations or scallops; annulets.

2. (Mollusca: Bivalvia) a. A lamellibranch gill in which the lamella are thrown into vertical folds; the apex is farthest from the interlamellar cavity. b. Fold or costa involving the entire thickness of the shell wall

See also: plication.