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Arcoida (phylum Mollusca, class Bivalvia) An order of sedentary, epifaunal bivalves in which the adults are byssate or free-living. Typically they are isomyarian, taxodont, and equivalved. The shells are normally trapezoid and composed of crossed-lamellar calcium carbonate. They have filibranchiate gills and flat cardinal areas on the dorsal margin. They first appeared in the Lower Ordovician.