Plants small to medium-sized, in loose to dense, rarely fragile tufts, green, yellowish, brownish, or golden green, with orange tinge due to leaves yellow-green and stems reddish. Stems creeping, ascending, or sometimes erect, densely or loosely terete- or subcomplanate-foliate, sometimes julaceous, regularly or irregularly pinnate, branches similar or more strongly complanate-foliate; central strand present; pseudoparaphyllia acute; axillary hairs of 2–3(–4) cells. Stem leaves erectopatent to erect, imbricate or falcate-secund, lanceolate to ovate, slightly to strongly concave, not to moderately plicate; base short-decurrent; margins serrate, serrulate, or subentire; apex gradually tapered or acuminate; costa to (20–)40–80% leaf length, thin or thick distally, terminal spine present or sometimes absent; alar cells subquadrate or short-rectangular, small, walls moderately thick; laminal cells elongate to linear, smooth or occasionally prorate on dorsal surface, walls moderately thick; basal cells undifferentiated or slightly broader than adjacent cells, walls moderately incrassate, region clear or opaque across base. Branch leaves smaller, somewhat narrower; costal abaxial surface more strongly serrate. Sexual condition autoicous (synoicous in B. fendleri); perichaetial leaf acumen reflexed. Seta red-brown to reddish orange, rough, smooth, or roughened proximally and smooth distally. Capsule inclined, horizontal, or pendent (sometimes erect in B. fendleri), red-brown, remaining partly greenish until almost mature, ovate, elongate, or cylindric, usually curved; annulus separating in fragments; operculum bluntly short-conic; peristome xerocastique, perfect to moderately reduced. Calyptra naked. Spores 9–21 µm.
Species 15–20 (6 in the flora).
Brachytheciastrum was recognized as an infrageneric group within Brachythecium, as the velutinum group or sect. Velutina De Notaris, until M. S. Ignatov and S. Huttunen (2002) found that it is more closely related to Homalothecium than to Brachythecium (in the strict sense) and segregated it as a genus. However, there is little in common between Brachytheciastrum and Homalothecium in morphology, except the tendency of the leaf marginal teeth to stand at a wide angle, often perpendicular to reflexed, as well as the rather thick-walled laminal cells. Both genera are among the most xerophytic in the family and are more diverse in western North America.
|1||Plants medium-sized, intense yellow-green to orangish; leaves falcate-secund; setae usually very strongly rough, sometimes rough proximally and almost smooth distally; capsules horizontal to somewhat pendent; leaves ovate-lanceolate, 1.3-2.3 × 0.3-0.8(-0.9) mm; mountains of w North America.||Brachytheciastrum leibergii|
|1||Plants small, rarely medium-sized, green to yellowish or brownish; leaves straight to falcate, occasionally circinate; setae rough to smooth; capsules horizontal, inclined, or erect, rarely pendent; leaves ovate to lanceolate, 0.5-2.5 × (0.1-)0.2-0.5(-0.9) mm; widespread||> 2|
|2||Sexual condition synoicous; capsules ovate-elongate to cylindric, erect to inclined; endostome basal membranes less than 1/4 endostome length, cilia short to reduced or occasionally as long as segments.||Brachytheciastrum fendleri|
|2||Sexual condition autoicous; capsules ovate, subcylindric, or elongate, usually inclined to horizontal, rarely erect; endostome basal membranes 1/3 endostome length or longer (except B. delicatulum), cilia as long as segments (except B. delicatulum)||> 3|
|3||Leaves ovate to broadly ovate-lanceolate; apices abruptly or rarely gradually short- acuminate; stems julaceous or subjulaceous.||Brachytheciastrum collinum|
|3||Leaves ovate, ovate-lanceolate, or lanceolate; apices gradually or rarely abruptly long-acuminate; stems not julaceous||> 4|
|4||Leaves 1.4-2.5 × 0.3-0.5(-0.7) mm; capsules inclined, sometimes slightly erect; endostome basal membranes 1/5-1/3 endostome length, cilia short to absent; Utah.||Brachytheciastrum delicatulum|
|4||Leaves 1-1.8 × 0.1-0.5(-0.6) mm; capsules horizontal to somewhat pendent; endostome basal membranes 1/3-1/2 endostome cilia as long as segments; widespread||> 5|
|5||Spores 15-21 µm; costae usually thin distally.||Brachytheciastrum trachypodium|
|5||Spores 10-14(-15) µm; costae often thick distally.||Brachytheciastrum velutinum|