| Diagnosis |
Shell relatively high and narrow in outline, with undulating commissure and strong posteroventral elongation. Shape often very irregular, due to the confined space in which the shell grows and to the effect of repair. Dorsal margin long and straight to slightly arched, more or less strongly expanded posteriorly in a wing-like ear that increases with growth and may attain a size double the length of shell. Anterior margin elongated, markedly sinuous dorsally and extending a little or not at all forward of umbones. Posterior margin concave near the posterior ear, then more or less parallel to anterior margin. Ventral margin roughly rounded. Umbones small, pointing at anterior end of dorsal margin. Outer surface with irregularly concentric lamellate processes, often encrusted with marine growths and corroded towards the umbones. Ligamental area with numerous transverse grooves (more than 20 in large specimens). Nacreous area of the inner side of shell more or less expanded ventrally, its margin often poorly defined. Colour: outside of shell bluish purple to almost black, often paler to whitish umbonally. Interior porcelaneous and similarly coloured on non-nacreous area; nacreous area much paler, with bluish purple hue. |
