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.

Take the shape of the normal leaves in the first place. These are cordate at their base and mainly inequilateral, but the general shape varies to a considerable extent. This variation is closely related to the position of the leaves on the twigs, and shows distinct indications of complying with the general law of periodicity.