(noun; Anglo-Saxon folde
, a fold): 1. A doubling or folding.
2. (Brachiopoda) A major external elevation of the valve surface, convex in transverse profile and radial from the umbo.
3. a. (Mollusca: Bivalvia) A broad undulation in the shell surface which is directed radially or comarginally. b. (Mollusca: Gastropoda) The spirally wound ridge on the interior of a shell wall
See also: columellar fold, parietal fold.