Krypt. Fl. Brandenburg 2: 920. 1906. -- Hypnum triquetrum Hedw., Spec. Musc. 256. 1801. -- Hylocomium triquetrum (Hedw.) B.S.G., Bryol. Eur. fasc. 49–51. 1852,
Plants to 15 cm long, in pale-green or yellowish-green wefts, irregularly branched. Leaves plicate and strongly squarrose, closely foliate, to 7 mm, 2.5–3: 1, ovate and gradually contracted to the acumen. Line of leaf insertion encircling more than 180 degree of stem circumference. Margins plane or nearly so throughout, not at all decurrent, entire to serrulate at base, serrulate to serrate above. Median cells smooth, 6–8 µm wide, linear, about 4–8: 1, thick-walled with lumen: wall ratio 2–3: 1, strongly pitted. Median basal cells orange to red-brown, broader and shorter than adjacent, more distal cells, thick-walled with lumen: wall ratio about 1: 1. Alar cells somewhat differentiated in a rounded rather pellucid pocket, short-rectangular, 2–3: 1, strongly pitted, to 10 µm broad. Costa double with forks separate from base, symmetrically inserted in center of leaf base, with both forks extending above the middle. Paraphyllia absent. Juvenile leaves orbicular, serrate on margin, closely overlapping bud with first such leaf lateral to bud. Axillary hairs 5–7 celled, to 250 µm with two basal cells brown and short, mostly offset from leaf insertion. Stem cross-section with a small central strand, without a hyaloderm, with a well marked stereome of pachydermous, pigmented cells, and with inner cortical cells larger and mostly rather thick-walled. Rhizoids very rarely produced on old and prostrate stems or on bases of perichaetia, to 15 µm in diameter at insertion, arising from area immediately lateral to alar cells at leaf insertions, red-brown, monopodially branched with branches short, smooth and rather thick-walled.
Dioicous with male plants similar to females. Perichaetia on old, erect axes, with bracts to 3.5 mm long, clasping at base but abruptly contracted into the squarrose and narrow acumen. Seta to 4 cm long, erect, redddish brown, smooth. Capsule red brown, inclined and arcuate, neither sulcate nor strangulate. Urn to 3 mm, oblong and asymmetric, about 2: 1. Exothecial cells isodiametric to short rectangular, to 25 µm broad, in regular rows, with lumen: wall ratio 4–6: 1. Stomata phaneroporous, small and vertical pored, restricted to neck. Annulus present, not revoluble. Operculum conic, much shorter than urn. Exostome to 500 µm long, almost smooth but with light horizontal striae throughout, red-brown. Endostome segments lightly papillose, perforate along keel, about as long as exostome with basal membrane about 1/2 of total length. Cilia 1–3, lightly papillose, somewhat nodose. Spores green, smooth to very lightly papillose, to 15 µm.
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