Hypnum filicinum Hedwig, Sp. Musc. Frond., 285, plate 76, figs. 5-10. 1801; Amblystegium tenax var. spinifolium (Schimper) H. A. Crum & L. E. Anderson; Cratoneuron filicinum var. aciculinum (Müller Hal. & Kindberg) Grout

Plants somewhat stiff. Stems with shoot and branch apices often pale; paraphyllia sometimes present only in youngest shoot portions; axillary hairs few, weak. Stem leaves gradually or suddenly narrowed to apex, plane or slightly concave; alar cells usually many, region large, usually strongly widened marginally, reaching from margin to costa or almost so; medial laminal cells 12-55 × 4-11 µm. Branch leaves usually narrower, more strongly falcate. Perichaetia with inner leaves gradually or suddenly narrowed to apex, plicate, margins denticulate or strongly so distally, apex acuminate, laminal cells smooth.

Moist or wet habitats in calcareous districts, rock, tree bases beside streams, springs, moist soil, calcareous fens; low to high elevations; Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., Que., Sask., Yukon; Ala., Alaska, Ariz., Ark., Calif., Colo., Idaho, Ill., Ind., Iowa, Mich., Minn., Mo., Mont., Nev., N.H., N.Mex., N.Y., N.C., N.Dak., Ohio, Oreg., Pa., S.Dak., Tenn., Tex., Utah, Wash., Wis., Wyo.; Mexico (Baja California, México, Oaxaca, San Luis Potosí); South America; Eurasia; Africa; Atlantic Islands; Pacific Islands (New Zealand).

Strikingly rigid plants of Cratoneuron filicinum with extremely strong, often long-excurrent costae are sometimes found in streams or under heavily dripping water; these were sometimes recognized as Amblystegium tenax var. spinifolium in North America. Such plants have narrowly triangular or narrowly ovate-triangular leaves gradually narrowed to an acuminate or often obtuse point. Ordinary C. filicinum leaves are sometimes found in parts of such plants, or shoots on old plants of this kind may have a normal C. filicinum appearance. Molecular studies show that such plants likely represent habitat modifications and should therefore not be recognized taxonomically (L. Hedenas 2011, 2012). Cratoneuron curvicaule (Juratzka) G. Roth was attributed to North America by H. A. Crum et al. (1973) as C. filicinum var. curvicaule (Juratzka) Mönkemeyer; the species has been considered a synonym of C. filicinum on several American web pages. However, as shown by Hedenas (2003, 2011), this is a distinct species that differs both morphologically and molecularly from C. filicinum. The occurrence of C. curvicaule in North America has not been confirmed, and the only North American specimens referred to this species (H. G. Simmons 584 [in part] and 585, S) belong to C. filicinum.