Plants: small to large, in loose to dense tufts or mats, green to rich golden or more rarely pale yellow and whitish. Stems: creeping, ± densely terete-foliate, regularly or more rarely irregularly pinnate, branches densely terete-foliate; central strand present; pseudoparaphyllia with apex acute to acuminate; axillary hairs of 3–6 cells. Stem: leaves erect-appressed when dry, erect, erect-spreading, or rarely erect-appressed when moist, imbricate, lanceolate or narrowly triangular, weakly to strongly concave, strongly longitudinally plicate; base not to broadly decurrent; margins serrulate or entire, teeth sometimes sharp, recurved, especially in alar region; apex gradually tapered or acuminate; costa ending in distal lamina, occasionally percurrent, moderate to stout, terminal spine present or absent; alar cells quadrate to short-rectangular, rarely elongate; laminal cells elongate to linear, walls moderately thick; basal cells small, as wide or slightly wider than more distal cells, walls strongly incrassate, region opaque across base. Branch: leaves not differentiated, or somewhat narrower. Sexual: condition dioicous or phyllodioicous; perichaetial leaf acumen straight or more rarely reflexed. Seta: red-brown, rough or rarely smooth distally. Capsule: inclined, horizontal, or erect, red-brown, cylindric or rarely ovoid-oblong, straight or ± curved; annulus separating by fragments or scarcely separating; operculum long-conic and gradually rostrate, sometimes conic; peristome xerocastique, then perfect or slightly modified, or hygrocastique. Calyptra: naked. Spores: 10–21 µm. North America, nw Mexico, Europe, w, c Asia, n Africa, Atlantic Islands, Mediterranean climate areas.
Species ca. 10 (9 in the flora). A. J. Grout (1928–1940, vol. 3), as well as many early botanists, accepted a narrow circumscription of Homalothecium, placing some species in the genus Camptothecium. These taxa differ by peristome reduction in Homalothecium; the peristome is perfect or only slightly reduced in Camptothecium. However, H. Robinson (1962) combined the two genera in Homalothecium, and this decision was followed by E. Lawton (1965, 1971) and subsequent American authors. Recent molecular phylogenetic studies (S. Huttunen and M. S. Ignatov 2004; Huttunen et al. 2008) demonstrated that peristomial reduction took place independently in more than one lineage of the genus. The genus was monographed by H. Hofmann (1998), and its phylogeny was reconstructed by Huttunen et al. The latter analysis demonstrated that Homalothecium megaptilum, treated by W. B. Schofield (1968) in the separate genus Trachybryum, is nested in the Homalothecium clade, supporting the opinions of D. H. Norris and J. R. Shevock (2004) and Ignatov and Huttunen (2002), who included it in Homalothecium. The recently described H. californicum has a number of common features with H. megaptilum, but as an unrelated species, is an example of convergent evolution, and thus provides additional evidence for merging Homalothecium with Trachybryum.