Bryologist 74: 167. 1971. -- Hypnum haldanianum Grev., Ann. Lyc. Nat. Hist. New. York 1: 275. 1825. -- Heterophyllium haldanianum (Grev.) Kindb., Can. Rec. Sci. 6: 72. 1894.
Plants in dense mats, glossy green to almost golden green, irregularly but rather densely branched with branches and stems little different. Leafy axes similar wet or dry, to 10 cm long, with closely imbricated leaves especially near the almost spinulose apex. Leaves ovate-lanceolate, to 1.8 mm, about 3: 1, deeply concavwe and homomallous, abruptly contracted near the apex into a short and straight acumen. Margins plane, not at all decurrent, entire throughout. Median cells smooth, to 7 µm wide, linear, up to 20: 1, thin walled, not pitted. Alar cells somewhat enlarged and thin-walled in an excavate pocket, short rectangular, to 18 µm wide, accompanied by several quadrate but not inflated supra-alar cells with moderately thick but unpitted walls. Costa inconspicuous, short and double. Pseudoparaphyllia abundant, lanceolate, often narrowly so, serrulate to entire, erect. Axillary hairs 4-celled, to 150 µm with a short basal brown cell, not offset from leaf insertion. Stem cross-section with a well-defined central strand, with the outer cortex composed of several layers of small and brownish pachydermous cells, and with inner cortical cells enlarged with thin and hyaline walls. Rhizoids mostly rather abundant on stoloniferous axes, yellow-brown to brown,
Autoicous with perigonia and perichaetia placed near one another in leaf axils on stoloniferous axes. Perichaetia somewhat longer than the subtending leaves, with the bracts to 2 mm long, elliptic-oblong below and closely sheathing the seta, abruptly contracted at 1/2–2/3 into a reflexed, somewhat flexuose acumen. Seta to 3 cm long, erect but somewhat flexuose, brown to red-brown. Capsule brown, cylindric and arcuate, often strongly so, neither sulcate nor strangulate. Urn to 3 mm, about 3–4: 1. Exothecial cells near mouth of capsule differentiated in only 1–3 rows, dark-brown, isodiametric to somewhat transversely elongate with thick walls and angular lumens, to 15 µm wide. Exothecial cells at mid urn irregular in size and shape, rectangular with oblique end walls, 2–8: 1, in regular rows, not collenchymatous, to 22 µm broad. Stomata mostly somewhat cryptoporous, restricted to base of urn. Annulus ill-defined, falling in fragments. Operculum conic, much shorter than urn. Exostome pale to yellow-brown, to 700 µm long, closely horizontally striate below, papillose above, with strong ventral trabeculae. Endostome segments papillose, perforate and keeled, about as long as exostome with basal membrane about 1/2 of total length. Cilia single to wanting, mostly shorter than the processes. Spores smooth to very lightly papillose, to 15 µm.