Hedwigia 46: 310. 1907. Amblystegium exannulatum (Schimp.) De Not., Atti Univ. Geneova 1: 142. 1869. Harpidium exannulatum (Schimp.) C. E. O. Jensen, Meddel. Groenland 3: 324. 1887. Drepanocladus exannulatus (Schimp.) Warnst., Beih. Bot. Centralbl. 13: 405. 1903. Hypnum exannulatum Schimp. in B.S.G., Bryol. Eur. 6: 110. 1854. Type: Germany.
Drepanocladus exannulatus fo. angustissimus Mönk., Laubm. Eur. 784, fig. 185, c. 1927.
Drepanocladus exannulatus var. angustissimus Mönk. in Herz., Bot. Jahrb. Syst. 47: 489. 1912, nom. nud.
Plants large, green, yellowish brown, or purple, in densely caespitose mats. Stems prostrate or ascending, to ca. 25 cm long, regularly or irregularly pinnately branched, branches usually secund, curved, rarely straight at tips; stem nearly rounded in cross section; cortical cells small, rounded-hexagonal, thin-walled, epidermal cells small, thick-walled or hyaline, inflated; central strand weakly differentiated. Leaves polymorphous, usually falcate-secund, ovate-lanceolate, 1.5–3.0 mm × 0.4–1.0 mm, sometimes plane, involute at apex; margins nearly entire or serrulate above; costae strong, 60–100 mm wide at base, subpercurrent, sometimes shortly excurrent; median leaf cells linear, 25–55 mm × 3.5–8.0 mm; alar cells often forming an obvious auricle, extending close to costa at leaf base, usually quadrate or rectangular, large, hyaline, thin-walled, or brownish, thick-walled, grouped in 1–2 rows at base. Dioicous. Setae 30–70 mm long, capsules 2–3 mm long; cilia 2–3. Spores 12–18 mm in diameter, papillose.