Hypnum brewerianum Lesq., Trans. Amer. Phil. Soc. 13: 12. 1865. -- Tripterocladium brewerianum (Lesq.) Fleisch., Laubm. Java 942. 1908. -- Bestia breweriana (Lesq.) Grout, Moss Flora N. Amer. 3: 10. 1928.
Isothecium howei Kindb., Rev. Bryol. 22: 82. 1895. -- Bestia breweriana var. howei (Kindb.) Grout, Moss Flora N. Amer. 3: 10. 1928.
Plants in very dense mats green to whitish-green, often with a russet cast. Stems to 8 cm long, prostrate, irregularly branched and markedly julaceous with somewhat ascending branches which curl when dry. Leaves of stems and branches similar, to 2 mm, 2–3: 1, symmetric, ovate to ovate lanceolate, widest near middle, abruptly acuminate, concave, densely imbricated, not much changed on drying. Margins plane or sometimes minutely recurved near insertion, not decurrent but usually minutely cordate at base, serrulate below the middle, but dentate distally with the teeth often multicellular. Median cells smooth, to 10 µm wide, 3–5: 1, moderately thick-walled with lumen/wall ratio to 3: 1, usually pitted. Alar cells quadrate to transversely elongate, very thick-walled (lumen/wall ratio 0.5–1:1), with intensely green contents, abruptly differentiated from adjacent laminal cells, extending up about 1/4 to 1/3 the margin of the leaf and reaching halfway to costa. Apical cells rhomboidal, 2–3: 1, only gradually differentiated. Costa extending about 3/4, usually ending in an abaxial spine. Pseudoparaphyllia lanceolate, sharply dentate and acuminate, to 100 µm long Paraphyllioid structures usually present near apices of branches, appearing as longitudinal ridges protruding as rounded, very green, structures to 6 cells high and about as wide. Axillary hairs 5-celled, to 120 µm with 2 basal brown cells, not offset from leaf insertion. Rhizoids red-brown, to 20 µm, smooth, in scattered fascicles on ventral surface of stem, especially near branch insertion.
Mostly dioicous with male plants similar to females, but sometimes autoicous with perigonia in leaf axils near perichaetia. Perichaetia on main axis, to 2.5 mm. Perichaetial bracts costate, abruptly acuminate with divergent acumens from a ligulate-lanceolate base; cells to 25:1, with straight lateral walls. Seta to 1.5 cm long, yellow- to red-brown, somewhat flexuose, smooth. Capsule pale brown but dark at base, not strongly strangulate Urn to 2 mm, 2–3:1. Exothecial cells irregularly arranged, to 25 µm wide, 1–2:1, with thick (lumen/wall ratio 3–4: 1), red-brown walls; stomata restricted to neck. Operculum conic to short-rostrate, about 1/2 as long as urn. Exostome to 500 µm long, horizontally striate below, papillose above, white. Endostome segments low papillose, open on keel, about as long as exostome with basal membrane about as high as the segments. Cilia 2, not appendiculate. Spores low papillose, 15–18 µm.