Planitivela planulata  

The overall shape is trigonal ovate. The anterior and posterior dorsal margins are straight, and the ventral margin is rounded. The valves are the same size and shape (equivalve) and there may no gape or a very narrow elongate gape at the posterior margin. The valves are also thin-walled and not very inflated. The lunule is lightly impressed, extends 3/4 of the anterior dorsal margin, and is defined by a fine, shallow groove. The umbones are central. The escutcheon is slightly impressed, but otherwise not well defined.

The external surface is white and can have light brown radial and/or comarginal stripes. The stripes can also be dark brown or purple. The interior color is white with purple coloration. The purple coloration can be concentrated at the posterior dorsal margin, the anterior edge of the ventral margin, the pallial line or the muscle scars.

The ligament is external, extends 1/5 of the posterior dorsal margin and is supported by nymphal ridges. On the left valve, there are three cardinal teeth. The anterior (2a) and posterior (4b) teeth are not bifid; the central tooth (2b) is bifid. There is also one anterior lateral tooth on the left valve. On the right valve, there are four cardinal teeth. The anterior tooth (3a) is very weak and not bifid, and the central tooth (1) is not bifid. The traditional posterior tooth (3b) has split into two teeth; the most posterior is partly fused with the nymph. There are also two anterior lateral teeth on the right valve. On both valves, there is a weak ridge running parallel to the posterior dorsal margin, but it is stronger on the right valve. The adductor scars are similar in area, but the posterior is more rounded than the anterior. The pallial sinus is rounded and extends to the posterior tooth in length.