Description: Thallus foliose, homoiomerous, subgelatinous when wet, membrane-like, rounded, closely adnate, broadly lobed, 2-7(-15) cm wide. Lobes 0.5-1.5 cm wide, 60-100 µm thick when moist, rounded at apex, with an entire, not swollen margin, dark olive-green to black, lighter and semi-translucent when moist, strongly radiately ridged and postulate, never isidiate. Lower surface paler green than upper surface, at times bluish green, with depressions corresponding to ridges, and scattered white hapters. Upper and lower cortices absent. Apothecia common, lecanorine, sessile and constricted at base, 0.8-1.5(-2) mm across, with an initially concave, finally convex, red-brown to almost black disc and a persistent, smooth thalline margin. Thalline exciple usually pseudocorticate; proper exciple euthyplectenchymatous to subparaplectenchymatous; hymenium colourless, 85-130 µm high, I+ blue; paraphyses coherent, simple or sparingly branched, 2-3 µm thick at mid-level, the apical cells swollen; hypothecium pale yellow. Asci 8-spored, clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores 4-5(-7)-septate, not constricted at septa, hyaline, broadly acicular and irregularly clavate with one end thicker, usually curved, (35-)40-75 x (5-)6-6.5(-7) µm. Pycnidia often numerous, immersed to semi-immersed, on both sides of thallus. Conidia straight, bacilliform or with slightly swollen ends, 4.5-6 x 1-1.8 µm Photobiont cyanobacterial (Nostoc, the cells in long chains). Spot tests: all negative. Chemistry: without lichen substances.
Note: a mild-temperate lichen found on the bark of more or less isolated broad-leaved trees, more thermophytic than the closely related C. nigrescens; presently extinct in northern Italy, except Liguria, still frequent and locally even common in humid sites of Tyrrhenian Italy, especially in Sardegna.