noun
bivalve
Refers to any mollusk with a two-part hinged shell, such as clams, oysters, and mussels.
アサリは二枚貝の一種です。
Asari clams are a type of bivalve.
巻貝 refers to gastropods (snails, conchs) with a single, usually spiral shell, while 二枚貝 refers to bivalves with two hinged shells.
Compound of 二枚 (two flat pieces) + 貝 (shellfish), literally 'two-piece shellfish'.