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Order, Dimyaria. Genus Megadesmus. Valves equal, inequilateral, thick, their edges even; umbones nearly central; hinge sunk, with an antiquated area and one ? or two ? large teeth in each valve; ligament external, large; impressions of the abducter muscles strong, nearly equal, united by the impression of the mantle, at the posterior extremity of which is a small shallow sinus; no lunette.