Meretrix casta
Similar to Meretrix astricta. Meretrix casta can be distinguished by its highly inflated umbones.
Similar to Meretrix meretrix. Meretrix casta is more inflated at the umbones and has a rounder posterior margin.
Similar to Meretrix ovum. Meretrix casta is distinguishable by its more robust shell and hinge plate, and its umbones project more.
Similar to Meretrix petechialis. Meretrix casta is distinguishable by its rounder shape and higher inflation at the umbones.
The overall shape is trigonal ovate. Both the anterior and posterior dorsal margins are slightly convex. The anterior, ventral and posterior margins are all rounded. The anterior half of the flank is slightly more inflated than the posterior. The valves are the same size and shape (equivalve), and there are no gapes when they are closed. Sculpture consists of growth lines. There may also be very fine radial lirae, most apparent on the central area of the flank. The lunule is lightly impressed, defined by a fine shallow groove, and extends 3/4 of the anterior dorsal margin. The umbo is slightly anterior, and the beaks also point slightly anteriorly. The escutcheon is lightly impressed, but otherwise not well defined.
The exterior surface is white, and it can have a light brown or dirty gray periostracum. There may be darker pigmentation surrounding the escutcheon. The interior is white with a posterior ray of dark purple coloration descending from the posterior end of the nymph, along the posterior margin.
The ligament is external, extends half the posterior dorsal margin and is supported by nymphal ridges. The hinge plates and teeth are very robust. On the left valve, there are three cardinal teeth. The anterior (2a) and central (2b) teeth may not be bifid or may be very lightly bifid. The posterior tooth (4b) is fused with the nymph; there are very fine vertical crenulations in the shallow groove between the nymph and the 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 tooth (3a) is weak and not bifid, the central tooth (1) is not bifid, and the posterior tooth (3b) is bifid. The nymph has fine vertical crenulations on its ventral side. There are also two anterior lateral teeth (A1, A3) on the right valve. The posterior adductor muscle scar is slightly more rounded than the anterior. The pallial sinus is very shallow.