Bryum hyperboreum Gunnerus ex Withering, Syst. Arr. Brit. Pl. ed. 4, 3: 811. 1801; Cynodontium hyperboreum (Gunnerus ex Withering) Hagen; Dicranum hyperboreum (Gunnerus ex Withering) Smith

Plants in compact, dark green tufts. Stems 1-3(-5) cm. Leaves erect-spreading, lanceolate, subulate, 2-3 mm; costa 30-55 µm wide at base, short-excurrent, rough near tip; distal laminal cells mostly subquadrate (1-2:1), incrassate; basal laminal cells elongate, alar cells differentiated, quadrate or slightly enlarged. Seta 4-6(-8) mm. Capsule exserted, slightly curved, obscurely to distinctly ribbed when dry, annulus developed, separating; peristome large, not spreading outward when dry. Spores 18-30 µm.

Capsules mature summer. Siliceous rock or soil; high-alpine elevations; Greenland; Europe.

Arctoa hyperborea is a rare arctic-alpine moss found on rock ledges or crevices at high elevations. It is distinguished from other species of the genus by its short-excurrent costa, shorter seta (the length is highly variable), and peristome not spreading when dry.