Description: Thallus foliose, homoiomerous, gelatinous when wet, 3-5 cm wide, deeply and broadly lobed. Lobes 2-6 mm wide, 0.1-0.25 mm thick, often repeatedly branched, broadly canaliculated, rounded at tips, the margins not swollen, coarsely undulate. Upper surface dark olive-green to black, paler and subtransparent when moist, often somehow pustulate, with dense, laminal, mostly globular isidia (up to 0.3 mm broad). Lower surface paler, with bundles of white hapters. Upper and lower cortex absent. Apothecia not always present, sessile, 0.5-1.5(-2) mm across, with a plane to convex, red-brown disc and a thick, entire, smooth or isidiate thalline margin. True exciple euparaplectenchymatous; hymenium colourless, 85-130 µm tall. Asci 8-spored, narrowly clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores submuriform or muriform, often with 3 transversal and 1 longitudinal septa, hyaline, ellipsoid to subglobose, 15-24(-28) x 6.5-13(-15) µm Pycnidia: laminal or marginal, immersed or semi-immersed, pale. Conidia bacilliform or with slightly swollen ends, 4.5-6 x 1.5 µm. Photobiont cyanobacterial (Nostoc), in long chains. Spot tests: all negative. Chemistry: without lichen substances.
Note: a widespread holarctic lichen found on calciferous rocks, more rarely on epilithic mosses, in moderately sheltered sites with some water seepage after rain, with a wide altitudinal range; one of the most common species of the genus in Italy.