Tivela fulminata
The overall shape is trigonal ovate with a rounded ventral margin. The anterior dorsal margin is slightly concave; the posterior dorsal margin is straight. Sculpture consists of growth lines and very fine comarginal ribs that become stronger anteriorly and posteriorly. The lunule is impressed and elongate, extending up to 3/4 of the anterior dorsal margin. The commissural margin is raised at the center of the lunule. The umbo is nearly central and points slightly anteriorly. The escutcheon is slightly impressed, but otherwise not well defined.
The exterior of the shell is white and can have a variety of brown coloration patterns. There may be brown comarginal stripes, radial stripes, mottling and/or tent marks. The brown color can also be concentrated on the posterior dorsal margin. The interior of the shell is white, and can have a tan semi-circular pattern of coloration.
The ligament is external, extends 1/3 of the posterior dorsal margin and is supported by rugose nymphal ridges. On the left valve, there are three cardinal teeth; the anterior (2a) and central (2b) teeth are not bifid, and the posterior tooth (4b) is bifid. There may be a projection on the nymph that appears to be a fourth cardinal tooth. There is also one anterior lateral tooth (A2) on the left valve. On the right valve, there are four cardinal teeth. The anterior (3a) and central (1) teeth are not bifid. The traditional posterior tooth (3b) has split into two teeth, and the most posterior is fused with the nymphal ridge. There are also two anterior lateral teeth (A1, A3) on the right valve. On both valves, there is a weak ridge running parallel to the posterior dorsal margin. The anterior and posterior adductor scars are similar in size and shape. The pallial sinus is rounded and extends to the end of the ligament in length.