Orthotrichum arcticum Schimper; O. arcticum subsp. blyttii (Schimper) Kindberg; O. arcticum var. groenlandicum (Berggren) Mönkemeyer; O. blyttii Schimper; O. blyttii subsp. arcticum (Schimper) Macoun; O. blyttii var. arcticum (Schimper) I. Hagen; O. blyttii var. groenlandicum (Berggren) C. E. O. Jensen; O. blyttii subsp. microblephare (Schimper) Macoun; O. blyttii var. microblephare (Schimper) C. E. O. Jensen; O. groenlandicum Berggren; O. idaohense Cardot & Thériot; O. pylaisii subsp. arcticum (Schimper) Kindberg; O. pylaisii subsp. groenlandicum (Berggren) Kindberg; O. pylaisii subsp. microblephare (Schimper) Kindberg; O. rhabdophorum Venturi

Plants to 3 cm, . Stem leaves stiff, erect-appressed when dry, ovate-lanceolate to lanceolate, 2-3 mm; margins revolute, entire; apex broadly acute to narrowly obtuse, often bluntly acuminate; basal laminal cells rectangular-elongate, walls thick, ± nodose; distal cells 9-16 µm, 1-stratose, papillae 1-4 per cell, 2-fid, small. Specialized asexual reproduction absent. Sexual condition gonioautoicous. Seta 2-3 mm. Capsule emergent or exserted, oblong to cylindric, 1.2-2.5 mm, slightly 8-ribbed distally; ; stomata superficial; peristome double or single; prostome occasionally present, small; exostome teeth 16, sometimes connate in 8 pairs, reflexed-recurved, densely papillose, ; endostome segments often absent, occasionally rudimentary, or 8, well developed, , of 1 row of cells, smooth or finely papillose. Calyptra oblong to oblong-conic, smooth, sparsely hairy, hairs smooth. Spores 16-27 µm.

Rock near oceans or alpine or arctic zones, on Salix or Alnus, bone, in or near bird rookeries, granitic boulders in coniferous forests; low to high elevations (100- 3000 m); Greenland; Alta., B.C., Man., Nfld. and Labr., N.W.T., Nunavut, Que., Yukon; Alaska, Calif., Idaho, Mont., Wash., Wyo.; n Europe; n Asia.

Orthotrichum pylaisii is distinguished by the ribbed capsules with reflexed exostome teeth and a rudimentary endostome, along with superficial stomata and large, ovate-lanceolate leaves. The capsule is usually exserted in North American populations; however, in European populations it is sometimes only emergent.