Description: Thallus small-foliose, homoiomerous, gelatinous when wet, forming up to 5 cm wide rosettes. Lobes adpressed, linear to fan-shaped, usually convex, dull, dark olive-brown to black, often minutely striated, repeatedly dichotomously branched, radiating, 0.5-1.5 mm wide, much swelling when wet (up to 350 µm thick), not pubescent, without isidia. Lower surface usually paler, with tufts of white hapters. Upper and lower cortices absent. Apothecia frequent, lecanorine, sessile, 1-2 mm across, with a flat to slightly convex, brown-black disc, and a usually thin, persistent, smooth or crenulate-lobulate thalline margin. Thalline exciple ecorticate; proper exciple thick, 100-150 µm wide laterally, euparaplectenchymatous; epithecium brownish; hymenium colourless, 90-120 µm high, I+ blue; paraphyses simple or sparingly branched in upper part, 2-4.5 µm thick at mid-level, the apical cells slightly wider; hypothecium pale brown, I+ blue. Asci 8-spored, narrowly clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores 3-4-septate, hyaline, linear-oblong, often curved at maturity, (20-)25-45(-60) x 4.5-6.5 µm. Pycnidia usually laminal, immersed, globose, pale with darker apex. Conidia bacilliform, straight, slightly swollen at both ends, 4.5-7 x 1-1.5 µm. Photobiont cyanobacterial (Nostoc, the cells in chains). Spot tests: all negative. Chemistry: without lichen substances.
Note: a mainly temperate to southern boreal-montane species found on calcareous rocks in rather sheltered situations; widespread, but never common.