Musculus discors

(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).

Up to 13 mm in length.

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.

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.

On rocky shores from mid-tidal zone into shallow sublittoral.

Distribution circumboreal, in the north-east Atlantic ranging south to the Mediterranean and Madeira (Distr. M. discors).