Megapitaria squalida
The overall shape is trigonal, ovate. The posterior margin is weakly pointed, and the anterior and ventral margins are rounded. The anterior dorsal margin is straight, and the posterior dorsal margin is subconvex. The valves are equal in size and shape (equivalve), and there is no gape when they are closed. Sculpture consists of growth lines only. The lunule is spear-shaped, extends half the anterior dorsal margin and is defined by a fine, shallow groove. The umbo is anterior, and the beaks also point slightly to the anterior. The escutcheon is not well defined.
The exterior is typically light brown with darker brown comarginal stripes, radial stripes and/or tent marks. There are typically two prominent, dark brown radial stripes. The comarginal stripes may also be purple ventrally. There are typically brown tent marks on the umbo, but they may also cover the entire flank. The periostracum is light brown or olive-brown. The interior is white, and there may faint purple coloration internally, especially along the some of the growth lines or outside of the pallial line.
The ligament is external, extends 1/3 of the posterior dorsal margin and is supported by nymphal ridges. The nymphs have a granulose texture. On the left valve, there are three cardinal teeth, none of which are bifid. The anterior (2a) and central (2b) teeth are joined dorsally, forming an inverted "v" shape. The posterior tooth (4b) is fused with the nymph. There is also one anterior lateral teeth (A2) on the left valve. On the right valve, there are three cardinal teeth: the anterior (3a) and central (1) teeth are not bifid, and the posterior tooth (3b) is bifid. There are two anterior lateral teeth (A1, A3) on the right valve, as well as a fine, weak ridge running parallel to the posterior dorsal margin. The anterior and posterior adductor muscle scars are similar in size and shape. The pallial sinus is deep, extends to the middle of the nymph in length, and its deepest point forms an acute angle.