Tivela stefaninii
Similar to Planitivela planulata. Tivela stefaninii is distinguishable by its thicker and more inflated shell, and it also lacks interior coloration. Planitivela planulata has a thinner and less inflated shell, and interior purple coloration.
Similar to Tivela ventricosa. Tivela stefaninii is distinguishable by its centered umbones and the even inflation between the anterior and posterior ends of its flank. Tivela ventricosa has slightly anterior umbones, as well as a more inflated posterior relative to the anterior end of the flank.
The overall shape is trigonal ovate, and the posterior margin is slightly more pointed than the anterior. The valves are similar in size and shape (equivalve) and there is no gape when they are closed. Sculpture consists of growth lines and extremely fine comarginal ribs that become more distinguished as the animal grows. The lunule is slightly impressed and is defined by a very fine, shallow groove. It can extend up to 2/3 of the anterior dorsal margin. The umbo is nearly central, and the beaks point to each other. The escutcheon is slightly impressed, but otherwise not well defined.
The exterior is white with a tan periostracum. There may be light brown comarginal or radial stripes. The escutcheon may be outlined by a light brown or light purple coloration. The interior is white.
The ligament is external, can extend up to 1/3 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, and the central tooth (2b) is bifid. The posterior tooth is rugose and fused with the nymphal ridge. 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 (1) and posterior (3b) teeth can be either not bifid or very slightly bifid. There are also two anterior lateral teeth (A1, A3) on the right valve. Both valves have a fine ridge running parallel to the posterior dorsal margin. The posterior adductor muscle scar is more rounded than the anterior, but both are similar in area. The pallial sinus is elongate and extends to the umbo in length.