Plants in very small tufts, golden--green distally, brownish proximally. Stems 0.5 mm wide and up to 3--4 cm long, sparsely branched, often with slender stoloniferous branches bearing minute leaves. Leaves closely appressed--imbricate, margins plane, broadly ovate, abruptly very short-acuminate to apiculate, 0.6--0.8 mm, slightly serrate above; median leaf cells oblong to oblong-rhombic, ca. 35--40 × 9 µm; basal cell shorter, incrassate and deeply colored, obscure; alar cells scarcely differentiated; costa lacking. Specialized asexual reproduction absent. Sporophytes unknown.

Wet calcareous habitats, arctic tundra; Greenland; N.W.T., Nunavut; Alaska.

Orthothecium acuminatum is distinguished by narrow stems 0.5 mm wide and up to 4 cm long, with straight, broadly ovate, abruptly short-acuminate to apiculate leaves. It is known in North America only from a few localities in the Northwest Territories and one locality in Alaska. William Weber (personal communication to R. R. Ireland) suggested that O. diminutivum (Grout) H. A. Crum & L. E. Anderson, described from Colorado, is synonymous with Isopterygiopsis puchella (Hedwig) Z. Iwatsuki. R. R. Ireland (2003) accepted this suggestion.