Abstract
Shell transversely subpentagonal, rounded, subglobose, unequally biconvex. Pediele-valve shallower than the brachial valve, with a broad, weak, undefined, median sinus, widening anteriorly; beak high, rounded, incurred, pointed; interior of valve with the walls and floor of the umbonal cavity thickened, with thin low dental plates fused to the sides and supporting small short teeth ; umbonal cavity sharply separated in front from the base of the deeply sunk subpyriform muscular area, extending about half the length of the valve, and composed of a pair of elongated flabelliform diductors enclosing very narrow shorter adductors. Bmchial valve deeper and more convex than the opposite valve, with a low, median, undefined, rounded fold; the interior of the valve has a long, lanceolate, well-defined muscular area extending fully three-fourths of the length of the valve, and composed of a pair of narrow adductors separated by a low, thick, median septum; hinge-plate thick, large, with a concave semicircular face divided by a narrow median slit into a pair of triangular areas; erura stout, rod-like, cylindrical, closely placed in the middle line. Posterior margins of both valves slightly thickened internally. Surface of valves covered with regular rounded fine thread-like riblets, 40 to 50 in number, 9 to 12 lying in the sinus. Dimensions.—Length=10 to 12 mm Width=12 to 14 ram. Remarks.—The material consists of several more or less perfect internal casts of complete shells, or single valves with impressions of the external surface of a few of the same specimens. The internal characters are well preserved,
Published Version
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