Description: Thallus foliose, heteromerous, dorsiventral, closely adpressed, up to 5(-7) cm broad, the lobes contiguous, grey to brownish grey, wrinkled, dull, 1-2(-3) mm broad, mostly convex, the apex much swollen and hollow inside, the surface largely covered by soredia arising from wrinkles on the upper surface, which crack and develop soredia along the edges of cracks. Lower surface black (brown at margins and lobe tips), rugose, without rhizines. Upper cortex of tightly packed, more or less anticlinally oriented hyphae, the cell walls with Cetraria-type lichenan; medulla soon becoming hollow, lining the cavity inside the lobes; lower cortex dark, paraplectenchymatous. Apothecia extremely rare, lecanorine. Epithecium brown; hymenium and hypothecium colourless. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-celled, hyaline, subglobose, 5-6 x 4-5 µm. Pycnidia immersed, black. Conidia rod-shaped to weakly bifusiform. Photobiont clorococcoid. Spot tests: upper cortex K+ yellow, C-, KC-, P+ pale yellow, UV-; medulla K-, C-, KC+ red, P-, UV+ violet-blue. Chemistry: upper cortex with atranorin and chloroatranorin; medulla with physodic acid (major), 3-hydroxyphysodic acid (accessory), 2'-O-methylphysodic acid (minor accessory), and other accessory unknown substances.
Note: a cool-temperate to boreal-montane lichen, most frequent in the Alps, much rarer in the mountains of southern Italy.
Growth form: Foliose, narrow lobed
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)