4. Arctoa Bruch & Schimp. in B. S. G., Bryol. Eur. 1: 151. 1846.
Plants small, yellowish green, in sparsely radiculose, more or less shiny tufts. Stems erect, simple or forked; central strand present. Leaves erect-spreading or falcate-secund, lanceolate, more or less channeled, rather long-awned; margins entire or serrulate only at the apex; costa single, narrow, filling most of the awn and excurrent, in cross section, median guide cells and stereids poorly developed, other cells thick-walled; upper cells rounded-quadrate or rectangular, thick-walled; lower cells becoming elongated; alar cells somewhat enlarged, quadrate or shortly rectangular. Autoicous. Perichaetial leaves sheathing at base, suddenly narrowed to a long awn. Setae 3–6 mm long, straight, stout, yellowish; capsules erect, obovoid, nearly symmetric, contracted below a wide mouth, tapered to the base, and somewhat furrowed when dry and empty; annuli compound; opercula obliquely rostrate; stomata present; peristome teeth 16, inserted near the mouth, lanceolate, entire or perforate, irregularly divided only in the upper half or less, vertically or irregularly striate nearly to the tips. Calyptrae cucullate, not fringed at the base, smooth. Spores spherical, 18–30 µm in diameter, finely papillose.
Gametophytically, Arctoa is similar to Dicranum in many aspects, but it differs in having a nearly homogeneous structure of the costa, which lacks stereids or only with poorly developed guide cells and stereids. The genus consists of about 5 species in the world. Two species are known from China.