Grimmia robusta Nees & Hornschuch in C. G. D. Nees et al., Bryol. Germ. 2(1): 123, plate 20, fig. 9. 1827

Plants in open to compact tufts, olivaceous, sometimes brownish or yellowish. Stems 1-5 cm, central strand distinct. Leaves erect or curved when dry, narrowly ovate-lanceolate to ovate-lanceolate, sharply keeled distally, 1.6-3 mm, 1-stratose, rarely with 2-stratose striae distally; margins recurved to near apex, smooth, 1- or 2-stratose; apices acute, rarely somewhat obtuse; costa usually long-excurrent as a spinulose-denticulate, usually decurrent awn, rarely percurrent, smooth; basal marginal cells quadrate or oblate, trigonous; distal laminal cells short-rectangular, occasionally isodiametric, 8-11 µm wide, strongly sinuose. Sexual condition autoicous. Capsule light brown or red-brown, cylindric to elongate-cylindric, (0.8-)1-1.6 mm, sometimes striated; exothecial cells mostly irregularly shaped, mainly elongate, with numerous isodiametric and, sometimes, a few oblate cells, walls thin-walled to unevenly thickened and somewhat curved, usually trigonous; stomata present; peristome patent to squarrose, sometimes twisted, 300-430 µm, red or orange-red, densely papillose, entire or weakly perforated. Spores 8-11 µm, nearly smooth.

Capsules mature late spring to early summer. Dry to periodically moist calcareous rock; moderate to high elevations (400-2100 m); Alta., B.C., Nfld. and Labr. (Nfld.), N.W.T., Ont., Que., Yukon; N.Dak., S.Dak., Wash.; Europe.

The long, spinulose-denticulate and decurrent awns, strongly sinuose laminal cells, and brown or red-brown, cylindrical capsules with primarily elongate exothecial cells are characteristic features of Schistidium robustum.