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Myoida (phylum Mollusca, class Bivalvia) An order of bivalves which are not equilateral in shape and in which the valves may be equal or unequal. The shells are thin and never nacreous. The dentition consists of a cardinal tooth in each valve, and some have edentulous valves. Musculature is isomyarian or anisomyarian, and there is no escutcheon or lunule. Siphons are well developed for a burrowing or boring way of life. They first appeared in the Carboniferous.