Fl. Franç. ed.3, 2: 491. 1805 ,
Species 24 (4 in the flora).
The North American species of Oligotrichum show wide variation in habit, leaf form, and development of abaxial lamellae. As in Pogonatum, the peristome teeth are compound (double), but in Oligotrichum the teeth are pale and not intensely pigmented. The indistinctly angled or ridged capsules of some species also resemble those of Pogonatum, but in that genus the capsules lack stomata and the exothecial cells are papillate. Oligotrichum parallelum is a lax plant, similar to Atrichum in appearance, but without a differentiated leaf border. All four species produce abaxial as well as adaxial lamellae, most notably O. aligerum.
|1||Leaves 3-6 mm, crisped when dry, laxly spreading when moist; leaf margins coarsely and often doubly serrate; adaxial lamellae straight, not or scarcely undulate; stems 2.5-6 cm.||Oligotrichum parallelum|
|1||Leaves 1.5-3 mm, not crisped when dry, not much different wet or dry; leaf margins various: regularly serrate, finely and distantly serrulate, or almost entire; adaxial lamellae tall and distinctly undulate; stems 1-2(-3) cm||> 2|
|2||Leaves narrowly lanceolate, channeled above and ± erect-appressed when dry; lamina and costa with prominent abaxial lamellae; leaf margins distinctly serrate.||Oligotrichum aligerum|
|2||Leaves ovate-lanceolate, subtubulose above, arcuate-incurved or distinctly falcate-secund; abaxial lamellae mostly restricted to the costa; leaf margins remotely denticulate or almost entire||> 3|
|3||Leaves ± straight, arcuate-incurved; costa with abaxial lamellae usually present, often extending to below mid-leaf||Oligotrichum hercynicum|
|3||Leaves strongly falcate-secund; costa smooth at abaxially or with low abaxial lamellae or teeth near apex.||Oligotrichum falcatum|