Mnium tetragonum Hedwig, Sp. Musc. Frond., 73. 1801; Conostomum boreale Swartz

Stems brown. Leaves stiffly erect, keeled, 1-1.5 mm; alar cells undifferentiated; basal laminal cells rectangular; distal cells 24-30 × 7-8 µm. Perigonia large, paraphyses subclavate. Perichaetial leaves subulate, base ovate, 2-2.5 mm. Seta yellowish red to brown, 0.8-2.5 cm. Capsule ovoid, slightly asymmetric, 2-2.5 mm, neck short, swollen; operculum obliquely rostrate, beak stout; exostome teeth with border narrow, yellow, connate teeth forming perforated cone. Spores 41-52 µm.

Capsules mature Jun-Aug. Thin soil, cliff ledges, among boulders, waterlogged soil of late season snow melt; low to high elevations (0-2200 m); Greenland; Alta., B.C., N.B., Nfld. and Labr., N.W.T., Nunavut, Que., Yukon; Alaska, Colo., Maine, Mont., N.H., N.Y., Oreg., Wash.; Europe; c, n Asia.

Conostomum tetragonum is easily recognized by its glaucous, stiffly erect leaves in five rows and apically connate peristome teeth. Philonotis seriata (known only from Greenland in the flora area), shares a pentastichous leaf arrangement with C. tetragonum, but the laminal cells of P. seriata are prorulose at the proximal ends and the peristome teeth are free.