Mnium tetragonum Hedwig
Plants in dense, glaucous, sometimes yellowish tufts. Stems simple or forked, erect, 0.5--3 cm. Leaves narrowly lanceolate, not clasping, 1--1.5 mm, stiffly erect in 5 rows; margins slightly revolute in distal half, serrulate near apex; costa stout, subpercurrent at least in proximal leaves to excurrent, rough abaxially distally; distal laminal cells rectangular to short-rhomboidal, thick-walled, 24--30 × 7--8 µm, smooth or mamillose from distal ends. Sexual condition dioicous, perichaetial leaves subulate from an ovate base, 2--2.5 mm. Seta 8--25 mm, yellowish red to brown, flexuose. Capsule ovoid with a short neck, somewhat furrowed when dry, 2--2.5 mm; operculum obliquely rostrate; peristome teeth elongate lanceolate, reddish orange to dark brown with a narrow yellow border, smooth throughout or finely papillose at base, perforate and connate at apex. Spores spherical to reniform, 41--52 µm, coarsely papillose.
Capsules mature June--Aug. Thin soil, cliff ledges or among boulders, often on waterlogged soils of late season snow melt; 0--2200 m; Greenland; Alta, B.C., N.B., Nfld., N.W.T., Nun., Que., Yukon; Alaska, Maine, Mont., Oreg., N.H., N.Y., Wash.; Europe; c. and n Asia.
This species is easily recognized by the glaucous, stiffly erect leaves in 5 rows and the apically fused peristome teeth. Philonotis seriata (known only from Greenland in the flora area), shares the pentastichous leaf arrangement, but the leaf cells of Philonotis seriata are prorulose at the proximal ends and the peristome teeth are free.