Orthotrichum affine Bridel, Musc. Rec. 2(2): 22. 1801.
Plants to 3 cm. Stem leaves erect-appressed and straight when dry, ligulate, elongate- to ovate-lanceolate, 2--4 mm, acuminate, sharply acute, obtuse, or rounded, sometimes broadly apiculate; margins recurved-revolute to just below apex, entire; distal laminal cells 9--13 µm wide, 1-stratose with 2--3 low, forked papillae per cell; basal laminal cells long-rectangular to elongate, nodose. Sexual condition goniautoicous. Seta 1--2.2 mm. Capsule 1/2 emergent to shortly exserted; urn oblong-cylindric when mature, narrowly-cylindric and constricted along entire length when old and dry, 1.6--2.5 mm, strongly 8-ribbed 2/3 entire length of capsule; stomates superficial; peristome double; endostome segments 8, rarely 16, 2 cells wide at base, reticulate-papillose; exostome teeth 8, erect when mature, reflexed-recurved when old, coarsely and obscurely papillose or sometimes papillose-reticulate. Calyptra conic-oblong, with a few papillose hairs. Spores 15--18 µm.
Deciduous trees, rarely on shaded rocks, shaded forests near stream beds and in canyons, usually in mountainous areas; 0--3000 m; Alta., B.C.; Calif., Colo., Idaho, Mont., Oreg., Utah, Wash; Europe; Asia; n Africa.
Orthotrichum affine, a species confined to the western portion of North America, is best recognized by the following combination of characters: superficial stomates; cylindric, strongly ribbed, almost exserted capsules; a well-developed, but not stout endostome; and a calyptra that is only sparsely hairy.