This little Mediterranean shellfish, called violet in Provence and biju in Languedoc is also sold by various other names, depending on the region:

They are found in shallow waters along the Mediterranean coast. They feed by filtering sea water. Shaped like a large potato, blackish-brown in color on the outside, they are covered with epibiotes (fixed organisms), hence their Latin name Microcosmus (or "little world.") Violets take in sea water through a top siphon and eject it through a side siphon. As it passes through, a branchial structure retains the vegetable and animal micro-organisms necessary for its nutrition, as well as oxygen. They range in size from 15-20 cm on average. 

In culinary terms, violets have an appealing briny taste. 

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Photo: Patrick Heurteau; Réserve Naturelle Marine de Cerbère - Banyuls