Hypnum sordidum Müller Hal., Bot. Zeitung (Berlin) 14: 457. 1856; Cratoneuron sordidum (Müller Hal.) Brotherus; Drepanocladus exannulatus var. mexicanus Cardot; D. tenuinervis T. J. Koponen

Plants medium-sized, sometimes small or large. Stems pinnate or irregularly pinnate. Stem leaves strongly falcate-secund, rarely weakly so, ovate, broadly ovate, or triangular-ovate, gradually narrowed to apex, ± strongly concave, 1.4-5.2 × 0.4-1.4 mm; base erect or erectopatent, insertion slightly curved; margins entire or weakly and obtusely denticulate; apex acuminate, acumen gradually differentiated, mostly furrowed; costa single, ending beyond mid leaf, mostly in acumen but not excurrent; alar region quadrate or transversely short-triangular, reaching from margin 40-60% distance to costa; ratio of medial laminal cell length (µm) to leaf length (mm) 23.3-36.5. Sexual condition dioicous.

Intermediately mineral-rich, meso- to eutrophic habitats, submerged in lakes, pools, ox-bow lakes, terrestrial wetland habitats, fens; low to high elevations; Greenland; Alta., B.C., Nfld. and Labr. (Nfld.), N.W.T., N.S., Nunavut, Ont., Yukon; Alaska, Calif., Ind., Iowa, Mass., Mich., Minn., Nev., N.J., N.Y., Okla., R.I., Wash., Wis.; Mexico; Central America; South America; Eurasia.

Drepanocladus sordidus and 7. D. latinervis are recognized by their almost always strongly falcate leaves, relatively small alar groups, and relatively strong, non-excurrent costae; for differences between the two species, see the discussion of the latter.