Palliolum tigerinum

(Müller O.F., 1776)

Description (shell):
Shell thin, broadly oval, the curved ventral margin constituting almost a semicircle. Almost equivalve, right valve with narrow byssal notch in anterior ear, few poorly developed teeth on ventral border. Inequilateral: both valves with prominent anterior ear and negligible posterior ear. Sculpture of very fine concentric and radiating lines, visible only with hand lens, shell otherwise appearing smooth; numerous fine ribs may develop in later growth stages, up to 50 in number, often of unequal width, margin finely crenulate as a result. Growth stages clear but not prominent. Anterior ears of both valves with about six bold ridges (P. tigerinum-drawing).

Up to 25 mm in length.

Colour very variable, from white or cream through light reddish-brown to purple, in radiating or concentric bands or zigzags, or in irregular blotches, or both combined. A common form has a broad median band extending from umbone to margin. Left valve frequently more highly coloured and patterned than right, which is often unicolorous.

Colour creamy white to red. Attached by a byssus when small, later free and capable of swimming by the opening and rapid closing of the valves. The muscle for closing the valves is very large and powerful. The foot is developed as a finger-like organ, occupied in the spinning of byssal threads, which pass through the ears. The mantle margin is double, the inner finely fringed and the outer edged with long tentacles, and at their base gleam a row of 10 brown ocelli (eye-spots) with golden pupils.

Distributed from lower shore to about 100 m, most frequent on coarser ground.

Widespread and common, especially on the British coasts (Distr. P. tigerinus). Distributed from Norway to the Atlantic coasts of Spain, Portugal, and Morocco.