Sunetta menstrualis
The overall shape is ovate. The posterior margin is truncated; whereas, the ventral and anterior margins are rounded. The anterior dorsal margin is slightly concave, and the posterior dorsal margin is slightly convex. The valves are equal in size and shape (equivalve), and there are no gapes when they are closed. Sculpture consists of growth lines and very fine radial lirae. The lunule is lightly impressed, defined by a fine shallow groove, and extends half the anterior dorsal margin. The umbo is slightly anterior, and the beaks point slightly anteriorly. The escutcheon is deeply sunken.
The exterior color is white or light tan, and there may be a combination of comarginal stripes, radial stripes and/or tent marks on the flank. Commonly, there are dark brown and purple comarginal stripes with brown tent marks overlayed. The interior is white and may have purple or pink coloration concentrated in the muscle scars and/or outside of the pallial line.
The ligament is external, extends 1/3 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 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. The anterior (3a) and central (1) teeth are not bifid; the posterior tooth (3b) is bifid and fused with the nymph anteriorly. There are also two anterior lateral teeth (A1, A3) on the right valve. There are crenulations on the posterior and ventral margins, which become weaker at the anterior. The anterior and posterior muscle scars are similar in size and shape. The pallial sinus is pointed and extends to the posterior end of the nymph in length.