Plants in dense tufts, dark green to yellowish brown, shiny. Stems (0.5-)1-2(-5) cm, branches simple, sparsely radiculose. Leaves lanceolate, subulate, erect-spreading, sometimes falcate-secund; margins erect, entire, or serrulate near the tips; costa excurrent as an awn, narrow, stereids poorly differentiated from median guide cells; distal laminal cells rectangular to subquadrate, smooth or slightly mammillose; basal laminal cells elongate, smooth, sometimes porose, alar cells sometimes differentiated and inflated. Perichaetial leaves with a sheathing base. Sexual condition autoicous. Seta solitary, 3-6 mm, erect, stout, yellow. Capsule erect, exserted or immersed in perichaetial leaves, symmetric, obovoid, constricted below mouth, furrowed when dry, urn 0.5-1.2 mm; operculum obliquely rostrate; peristome single, of 16 red-brown teeth, divided halfway into two segments, vertically or irregularly striolate. Calyptra cucullate, smooth. Spores spheric, 16-30 µm, finely roughened, green. A rare northern and alpine genus, Arctoa occurs on rock or soil and is distinguished by its medium-sized, Dicranum-like habit, with poorly differentiated stereid and guide cells. It may be confused with Kiaeria, which differs by a longer seta, and capsules often strumose, with narrow mouths.
Allen, B. H. 1998. The genus Arctoa (Musci: Dicranaceae) in Maine. Evansia 15: 131-137.