Tivela ventricosa
Similar to Tivela stefaninii. Tivela ventricosa is distinguishable by its slightly anterior umbones, as well as its more inflated posterior end of the flank relative to the anterior. Tivela stefaninii has centered umbones and even inflation between the anterior and posterior ends of its flank.
The overall shape is trigonal ovate with a rounded ventral margin. The posterior margin is slightly more pointed than the anterior. The anterior dorsal margin is slightly concave; the posterior dorsal margin is straight. The posterior end of the flank is slightly more inflated than the anterior. The valves are the same size and shape (equivalve), and there is no gape when the valves are closed. Sculpture consists of growth lines and very fine comarginal ribs that can be more prominent at the posterior. The lunule is lightly impressed, defined by a fine shallow groove, and spans up to 2/3 of the anterior dorsal margin. The commissural margin is raised at the lunule; this results in the appearance of a second, raised lunule shape within the lunule. The umbones are slightly anterior. The escutcheon is lightly impressed, but otherwise not well defined.
The exterior color is white with a yellow or brown periostracum. There may also be light brown tent patterns, radial stripes and/or comarginal stripes on the shell flank. The interior color is white.
The ligament is external, spans up to 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 tooth (2a) is not bifid, the central tooth (2b) is lightly bifid, and the posterior tooth (4b) is bifid and has a rugose texture similar to the nymph. Rugose projections on the nymph may appear 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 tooth (3a) is weak and the central tooth (1) is weakly bifid. The traditional posterior tooth (3b) has split into two teeth; the most posterior is rugose and partially fused with the nymph. There are also two anterior lateral teeth (A1, A3) on the right valve. There is a weak ridge running parallel to the posterior dorsal margin on both valves, but it is stronger on the right valve. The adductor muscle scars are similar in area, but the anterior is more elongate than the posterior. The pallial sinus is rounded and extends to the end of the nymph in length.