Tivela byronensis
Similar to Tivela mactroides. Tivela byronensis is distinguishable by its distribution in the Pacific, from lower California to northern Peru (Olsson 1961). Tivela mactroides is found from the Carribean to Santa Catarina, Brazil (Rios 2009).
The overall shape is trigonal ovate. The valves are similar in size and shape (equivalve), and there is no gape when they are closed. Sculpture consists of growth lines and very weak, fine comarginal ribs. The lunule is elongate and can cover the anterior dorsal margin. The valves form a small ridge at the anterior dorsal margin, within the lunule. The beak points slightly anteriorly. The escutcheon is elongate, with a width half of the length, and can be as long as the posterior dorsal margin.
The external color is white or light tan. Other external coloration varies. There may be radial and comarginal light brown stripes. The lunule may have a central coloration of brown surrounded by white. The escutcheon is sometimes brown or white. The internal color is white.
The ligament is exterior and supported by nymphal ridges. There are four cardinal teeth on each valve. On the right valve, the anterior (3a) and central (1) teeth are not bifid; the traditional posterior tooth (3b) is split into two, and the most posterior tooth has a rugose texture. The right valve also has two anterior lateral teeth (A1, A3). On the left valve, the anterior (2a) and central (2b) teeth are not bifid; the traditional posterior tooth (4b) is split into two, and both halves have a rugose texture. The left valve has one anterior lateral tooth (A2). Both valves have a very fine ridge parallel to the posterior dorsal margin. The anterior and posterior adductor muscle scars are similar in size, but the posterior scar is more rounded. The pallial sinus is rounded and shallow.
Tivela byronensis is found from southern California to northern Peru (Olsson, 1961).