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Unionoida (phylum Mollusca, class Bivalvia) An order of unequilateral, and generally equivalved, marine and non-marine bivalves whose shell shape and size is very variable and whose ornamentation is usually simple. The musculature is isomyarian to sub-isomyarian. The ligament is external and opisthodetic. The dentition is quite variable, forms having one or two teeth, some pseudotaxodont and some edentulous. They first appeared in the Upper Devonian.

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