Arctica islandica

BéarlaShell: moderately swollen, thick and robust valves, almost circular, equivalved and inequilateral; has a thick dark brown to blackish periostracum; sculpture consists of thin concentric grooves; well-developed external ligament, reaching the end of the posterior margin of the valve, posterior to the umbones; hinge is heterodont, right and left valves each has three cardinal teeth, anterior and posterior lateral teeth; inside the valves are two slightly different muscle scars (dimyarian, anisomyarian), linked by a continuous pallial line (integropalliate), without a sinus; crossed-lamellae structure. Body: pair of gills each consists of two series of lamellae extensively linked by interlamellar junctions (eulamellibranch); foot is large and the two mantle lobes fuse to form two stout siphons.

• Cyprina islandica Linnaeus, 1767