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DescriptionA small bivalve, usually up to 12 mm in length. The shell is brittle, equivalved but inequilateral and rhomboidal in outline. The beaks and umbones slightly raised and a short distance from the anterior end. The anterior and posterior portions of the shell bear different numbers of radiating ribs, while the middle region lacks ribs. Where the ribs meet the shell margin, the margin is crenulated but is smooth elsewhere. The shell is yellow to brownish in colour with a pale green to olive periostracum. The inside of the shell is pearly (nacreous) with a narrow pallial line and distinct anterior and posterior adductor scars. This genus is unusual in that the byssus threads used to fix it to the substratum are woven into a nest or cage, that may incorporate macroalgae (e.g., Fucus spp. and Laminaria spp.). This species lays its eggs in mucus strings that are retained within the byssal nest.The nest completely encloses the adult so that Musculus discors is only visible when its valves are open and it is feeding. Smooth specimens, lacking ribs, were reported from Oban and Staffa, in the Hebrides (Jeffreys, 1863).