Ptychobranchus occidentalis shell description:

“Shell oblong, ovate, elliptical or subrhomboid, sometimes subtriangular, solid, scarcely inflated, rather wider behind; posterior ridge feeble, rounded or slightly double, ending behind in a faint biangulation; beaks low and not inflated, their sculpture not seen; surface nearly smooth, yellowish-green covered with delicate, wavy, green, faint rays; epidermis silky when fresh, but nearly smooth when rubbed; hinge solid; left valve with two low, small, solid pseudocardinals, the hinder larger and sometimes dentilate, with two remote laterals; right valve with one pseudocardinal and a vestigial one on each side of it, with one, sometimes more or less double, lateral; beak cavities shallow; muscle scars smooth and impressed; nacre bluish-white, often with lurid blotches thicker in front ”(Simpson, 2014; Mather, 2007).