1. Consisting of two plates; such as, the shells of bilaterally symmetrical molluscs which are laterally flattened and have shells made of two hinges: "Some examples of bivalves include clams, oysters, mussels, scallops, and cockles which are characterized by laterally flattened bodies and shells that consist of two hinged parts."

"Their enlarged gils are covered with cilia (fine hairs) and they have the additional functions of filtering microscopic food particles from the water as it flows over them."

"Bibalves life on the bottom of the sea or lakes and are sedentary or not moving around."

valva, "section of a folding or revolving door"; literally, "that which turns"; related to volvere "to roll".

The meaning was extended in about 1659 to "a mechanical device that works like a valve"; and in 1661 in zoology to "halves of a hinged shell".

An oyster is a bivalve because it has a two-valved shell.