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Anomalodesmata (phylum Mollusca, class Bivalvia) A subclass of equivalve, or nearly equivalve, bivalves most of which are relatively elongate. They have an aragonitic shell and many genera have a nacreous endostracum. If present, the ligament is opisthodetic, and the musculature is nearly isomyarian. There is a thickened hinge margin, and dentition is desmodont or edentulous. Living members have eulamellibranchiate, or occasionally septibranchiate, gills. The subclass first appeared in the Middle Ordovician. There is only one order, the Pholadomyoida.