Description: Thallus small-foliose, homoiomerous, gelatinous when wet, forming rosettes to 5 cm diam. Lobes adpressed, linear, usually convex, dull, dark olive-brown to black, often minutely striated, repeatedly dichotomously branched, radiating, 0.5-1.5 mm broad, much swelling when wet (to 350 µm thick), not pubescent. Isidia absent. Lower surface usually paler, with tufts of white hapters. Upper and lower cortices absent. Apothecia frequent, sessile, 1-2 mm across, with a flat, brown-black disc, and a usually thin, crenulate thalline margin. Proper exciple thin, paraplectenchymatous; hymenium brownish in upper part, otherwise colourless. 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, fusiform, often curved at maturity, 25-45 x 5-6 µm. Pycnidia usually laminal, immersed, globose, pale with dark apex. Conidia bacilliform, straight, slightly swollen at ends, 5-7 x 1-1.5 µm. Photobiont cyanobacterial (Nostoc), 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.