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(Risso, 1826)

Description (shell):
Shell thin, elongate oval, dorsal margin curving gently above umbones, posterior end appearing almost quadrate. Umbones subterminal, distinct. Often with broad streaks or zigzags, particularly anteriorly, and typically with reddish-brown chevrons along hinge line. About 10 broad, radiating ribs anterior to umbones, 20-30 posteriorly (thin dorsally, but broad and flat close to posterior margin). Umbonal ridges without ribs. Fine concentric lines also present, not markedly decussate where they intersect ribs; growth stages clear. Margin strongly crenulate in ribbed areas. External sculpture visible on inner surface.

Size:
Up to 13 mm long.

Colour:
Shell off-white, periostracum variably coloured: light or dark greenish brown, with patches of deep brown, reddish brown, or purple, umbones frequently darker. Ventral and posterior margins often marked with a broad band of deep green. Inner surface of shell pearly, sometimes tinted brown.

Animal:
The mantle forms a distinct incurrent tube in front and an excurrent one behind. The foot is strap-shaped and may be extended until it is a mere thread two or three times the length of the shell.

Habitat:
On rocky shores, lower shore and shallow sublittoral.

Distribution:
Ranging south to the Mediterranean and north-west Africa (Distr. M. costulatus).

Musculus costulatus