Callista umbonella
The overall shape is trigonal, ovate. The anterior, ventral and posterior margins are rounded. The anterior dorsal margin is subconcave, 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, and there may be very weak, dense comarginal ribs ventrally. The lunule is spear-shaped, defined by a fine, shallow groove and extends half the anterior dorsal margin. The umbones are prominent and anterior, and the beaks also point slightly to the anterior. The escutcheon is not well defined.
The exterior is white and there may be faint, light brown zig zag patterns on the umbo and purple coloration at the posterior margin. The lunule may be partially covered by purple pigmentation. The interior is white with a ray of purple coloration at the posterior, overlapping the posterior adductor muscle scar.
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 anterior (2a) and central (2b) teeth are joined dorsally, forming an inverted "v" shape. The posterior tooth (4b) is fused with the nymph; there are fine, vertical crenulations in the groove between the nymph and appressed tooth. 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, and the posterior tooth (3b) is lightly bifid. There are two anterior lateral teeth (A1, A3) on the right valve, as well as a fine, weak ridge that runs parallel to the posterior dorsal margin. The anterior and posterior adductor muscle scars are similar in area, but the posterior is slightly more rounded, while the anterior is more elongate. The pallial sinus extends to the posterior end of the nymph in length, and it may form an acute angle at its deepest point.