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Pterioda (phylum Mollusca, class Bivalvia) An order of bivalves that are byssate or cemented, and generally pleurothetic. Normally they are un-equilateral and unequivalve, and their variable shell composition may be crossed-lamellar, prismatic-nacreous, or foliate. The musculature is anisomyarian or monomyarian. The ligament is extremely variable. The gills are eulamellibranchiate or filibranchiate. The order first appeared in the Ordovician.