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. Phenology: 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.