Sunetta effossa
The overall shape is elongate, ovate. The posterior is subtruncated, and the anterior and ventral margins are rounded. The anterior dorsal margin is nearly straight, and the posterior dorsal margin is subconvex. The posterior half of the flank is slightly more inflated than the anterior half. The valves are equal in size and shape (equivalve), and there is no gape when they are closed. Sculpture consists of growth lines and wide, slightly rounded comarginal ribs. The ribs become wider as the animal grows. There are no ribs on the lunule or escutcheon. The lunule is elongate, lightly impressed and defined by a fine, shallow groove; it extends 1/2 of the anterior dorsal margin. The umbo is centered, and the beaks point slightly to the anterior. The escutcheon is deeply sunken and extends 3/4 of the posterior dorsal margin.
The exterior is white or cream with faint brown zig-zag patterns covering the flank. The patterns may be slightly mottled, especially ventrally. The lunule is marked by dark brown pigmentation and the escutcheon is covered by darker brown zig-zag patterns. The interior is white and there is typically purple coloration in the umbonal cavity, descending ventrally.
The ligament is external and short; it extends 1/4 of the posterior dorsal margin and is supported by nymphal ridges. On the left valve, there are three cardinal teeth (2a, 2b, 4b), none of which are bifid. The anterior tooth (2a) is weak and the posterior tooth (2b) is robust; they are fused dorsally, forming an inverted "v" shape. The posterior tooth (4b) is fused with the nymph. There is also one anterior lateral tooth (A2) on the left valve. On the right valve, there are three cardinal teeth (3a, 1, 3b), none of which are bifid. There are also two anterior lateral teeth (A1, A3) on the right valve. There are fine, antimarginal crenulations on the posterior and ventral margins, and they become weaker and fade at the anterior margin. The anterior and posterior adductor muscle scars are similar in size and shape. The pallial sinus is rounded, directed slightly dorsally, and extends to the posterior end of the nymph in length.