Clams and oysters, e.g. (8)
I believe the answer is:
bivalves
(Other definitions for bivalves that I've seen before include "Oysters, clams and others with hinged shell" , "Mollusks with two shells hinged together" , "eg Oysters" , "Shellfish with hinged double shells, like oysters" , "Molluscs with hinged shells" .)