(Linné, 1767)
Description (shell):
Shell thin, fragile, oval to rhomboidal; dorsal margin curving well above umbones, posterior broadly rounded, not tapering. Umbones subterminal, distinct. Sculpture of 8-12 broad ribs anteriorly, 30-50 more slender ribs posteriorly, and a few fine concentric lines; not decussate, margin of ribbed areas crenulate. External sculpture visible on inner surface (M. discors-drawing).
Size:
Up to 13 mm in length.
Colour:
Shell light yellow-brown, periostracum light brown, with a pale green tinge, often darker along ventral margin. Inner surface of shell pearly, sometimes with a bluish tinge.
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. The animal spins together branches of small seaweeds and dwells in safety within the nest.
Habitat:
On rocky shores from mid-tidal zone into shallow sublittoral.
Distribution:
Distribution circumboreal, in the north-east Atlantic ranging south to the Mediterranean and Madeira (Distr. M. discors).