Description: Thallus foliose, heteromerous, dorsiventral, adnate to loosely adnate, forming 1-10(-20) cm wide, orbicular rosettes. Lobes elongate, (0.25-)1-2.5 mm wide, irregularly divided and branched, slightly concave to flat, with thickened and somewhat raised margins. Upper surface dark brown to black (greenish brown in shade-forms), more or less shiny, with rounded to elongate, whitish, mostly marginal pseudocyphellae; lower surface dark brown to black, matt, somewhat paler at margins, with scattered, mostly simple, black rhizines. Upper cortex paraplectenchymatous, with a non-pored epicortex, the cell walls containing isolichenan; medulla white. Apothecia lecanorine, mostly submarginal, up to 9 mm across, with a dark disc and a usually knobby and pseudocyphellate thalline margin. Epithecium brownish; hymenium and hypothecium colourless. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-celled, hyaline, ellipsoid, 7-10.5 x 4.5-6 µm. Pycnidia frequent, mostly marginal, globose to elliptical, sessile or on up to 0.3 mm tall projections, often grouped into gall-like areas. Conidia bifusiform, dumbbell-shaped (thickened at both ends), 3.5-6(-7) x 1-1.5 µm. Photobiont chlorococcoid. Spot tests: upper cortex K-, C-, KC-, P-, N-, UV-; medulla K+ yellow turning reddish brown, C-, KC-, P+ yellow-orange, I-. Chemistry: stictic and norstictic acids (major), cryptostictic, menegazziaic and constictic acids (minor).
Note: a circumpolar, arctic-alpine lichen found on hard siliceous rocks wetted by rain, with optimum above treeline; somehow less bound to cold-humid sites than the similar but unrelated Cetraria commixta; widespread in the Alps, much rarer in the high Mediterranean mountains.