Orthorhynchula

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Treatise on Invertebrate Paleontology, Part H, Vol. 4 (2002):

• Transversely ovate to subcircular outline and dorsibiconvex profile. Beak suberect; ventral interarea prominent; delthyium open. Fold and sulcus moderate, from umbones; anterior commissure uniplicate; tongue low, serrate. Costae rounded, simple, from beaks. Dental plates fused to valve walls; ventral muscle field quadrate. Very short septalium supported on median ridge; hinge plates divided, prolionged anteriorly, concave ventrally; cardinal process septiform to lobate; crural bases subhorizontal .

Hall and Clarke(1893):

• “Shell rhynchonelloid in contour; hinge line short, stright, extending for about one third the transverse diameter of the valves. A true cardnial area is present on both valves, that of the pedicle-valve being considerably the broader, erect, often incurved. Each valve also possesses a distinct triangular delthyrium, that of the pedicle valve, according to the evidence at hand, never being in any degree closed by the deltidial plates. External surface strongly and simply plicated, the median fold and sinus being well developed. On the interior, the pedicle valve possesses blunt teeth which rest upon the laterally thickened walls of the valve and are not supported by lamellae. Between, and slightly in front of these lies a short, subquadrate muscular scar. The brachial valve possesses a linear cardinal process, on either side of which are two discrete crural plates, sharply concave on the upper surface and diverging anteriorly for a considerable distance. Shell-substance fiberous, impunctate.”

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