Description: Thallus foliose, homoiomerous, gelatinous when wet, forming 2-6(-10) cm wide rosettes, deeply lobed. Lobes radiating, elongated, 1-2.5 mm wide, 0.20.3(-0.45) mm thick (when wet), contiguous, adpressed for most of their length, ascending or erect towards the center, often channeled towards apices, the tips much swollen and plicate. Upper surface dark olive-brown to black, dull, without isidia. Lower surface usually somewhat paler with tufts of white hapters. Apothecia crowded towards thallus center, terminal and marginal, sessile to stipitate, 0.5-1.5 mm wide, with a plane to slightly convex, red-brown to (rarely) black disc, and a rather thin, smooth thalline margin. Proper exciple rather thin, subpara- to small-celled euparaplectenchymatous; hymenium colourless, 65-110 µm tall. Asci 8-spored, subcylindrical to narrowly clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores (2-)3(-4)-septate, hyaline, fusiform, with acute ends, (13-)18-28(-30) x (5-)6.5-8.5 µm. Pycnidia laminal or marginal, immersed in warts, globose, pale. Conidia straight or slightly curved, bacilliform, usually with swollen ends, 5-7 x 1-1.5 µm. Photobiont cyanobacterial (Nostoc), in long chains. Spot tests: all negative. Chemistry: without lichen substances.
Note: a widespread holarctic species found on exposed, hard, calciferous rocks and dolomite. Several south Italian records could refer to subsp. corcyrense.