Merriam-Webster Collegiate Dictionary - lamellibranch

 noun  (plural -branchs)  Etymology: New Latin Lamellibranchia, from lamella + Latin branchia gill  Date: 1855 any of a class (Lamellibranchia) of bivalve mollusks (as clams, oysters, and mussels) that have the body bilaterally symmetrical, compressed, and enclosed within the mantle and that build up a shell whose right and left parts are connected by a hinge over the animal's back  • ~ adjective