Physical Description

Morphology

The overall shape is elongate, ovate. In some specimens, specifically those from San Diego, California, the shape may be less elongate (Huber 2010). The posterior margin is subtruncated, and the anterior and ventral margins are rounded. The anterior dorsal margin is subconcave, and the posterior dorsal margin is subconvex. The valves are equal in size and shape (equivalve), and there is no gape when they are closed. Sculpture consists of growth lines and rounded comarginal ribs. The width of the interspace between two ribs is equal to the width of one rib. The ribs may also split; typically, they split posteriorly, resulting in more ribs at the posterior than the anterior, but they may also split anteriorly. The lunule and escutcheon lack ribs. The lunule is spear-shaped, extends 1/3 of the anterior dorsal margin and is defined by a fine, shallow groove. The umbo is anterior, and the beaks also point slightly anteriorly. The escutcheon is lightly raised, defined by a fine, shallow groove and extends 2/3 of the posterior dorsal margin.

The exterior is white. There may be some red-brown coloration at the umbo. The periostracum is light brown. The interior is white.

The ligament is external, extends half of the posterior dorsal margin and is supported by nymphal ridges. The nymphs have a rugose texture due to the presence of irregular, antimarginal pits. On the left valve, there are three cardinal teeth, none of which are bifid. The anterior (2a) and central (2b) teeth are joined dorsally, forming an inverted "v" shape. The posterior tooth (4b) is fused with the nymph. There is also one anterior lateral tooth (A2) on the left valve. On the right valve, there are three cardinal teeth: the anterior (3a) and central (1) teeth are not bifid, and the posterior tooth (3b) is bifid. There are two anterior lateral teeth (A1, A3) on the right valve, as well as a fine, weak ridge that runs parallel to the posterior dorsal margin. The posterior adductor muscle scar is slightly larger and rounder than the anterior. The pallial sinus is deep, tapered to a point, and extends to the middle of the hinge plate in length.

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© Anderson, Katherine L.

Source: Molluscs - eBivalvia LifeDesk

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