The European Cowrie, Trivia monacha, is common on rocky areas of the shore and in shallow water. Its shell is often enveloped by the animal's mantle when it is active, giving it a diffuse, soft outline. However, in this picture, the mantle is only partially raised, allowing a view of the deeply ridged shell.
The shell usually has three dark spots along the dorsal area but they may be indistinct, particularly with immature animals. A similar species, the arctic cowrie, has no spots. Cowries are very small, maybe just over 1cm long when inactive, and feed on sea squirts.
North Sea - St. Abbs Marine Reserve