I. ˈbī-ˌvalv adjective
Date: 1661
: being or having a shell composed of two valves
II. noun
Date: 1683
: any of a class (Bivalvia syn. Pelecypoda) of typically marine mollusks (as clams, oysters, or scallops) that have a 2-valved hinged shell, are usually filter feeders, and lack a distinct head