Weissia macrocarpa Hooker in T. Drummond, Musc. Amer., 74. 1828; Bryum porsildii (I. Hagen) C. J. Cox & Hedderson; Mielichhoferia macrocarpa (Hooker) Bruch & Schimper ex A. Jaeger & Sauerbeck

Plants pale light green to pink-green, red-brown to brown proximally. Stems 0.5-3 cm, not julaceous, weakly to strongly branched. Leaves flexuose to weakly secund when dry, erect when moist, ovate to oblong or broadly lanceolate, 0.6-2 mm; margins narrowly revolute proximally; apex broadly acute to sometimes acuminate, apiculus absent or rarely present; costa subpercurrent to percurrent, rarely short-excurrent to excurrent in hair-point, pale tan-brown or red; laminal cells 40-120 × 10- 18 µm; distal cells at apex elongate. Seta yellow-brown, 0.4-1.2 cm, curved to flexuose. Capsule erect to nutant, subglobose to short-pyriform, 1.3-2.5 mm; operculum flat to weakly convex, weakly apiculate; peristome single. Spores 12-20(-24) µm, smooth or finely papillose.

Capsules mature Jul-Aug (summer). Damp to seepy rock faces, crevices, limestone, arctic-alpine; low to high elevations (0-3500 m); of conservation concern; Greenland; Alta., Nfld. and Labr. (Nfld.), Nunavut; Alaska, Colo., Mont., Utah; Eurasia (ne Russia).

Although Haplodontium macrocarpum is associated with mineral-rich rock, collections come from a wide variety of rock types. In the southern and central Rocky Mountains, the species is apparently restricted to limestone. Some specimens from the Aleutian Islands have a strongly excurrent costa, and in some respects approach the Asian Mielichhoferia (Haplodontium) himalayana Mitten.