Description: Thallus foliose, homoiomerous, subgelatinous when wet, 1-5(-10) cm diam., adnate, broadly lobed. Lobes 0.5-1 cm wide, (60-)90-150 µm thick when wet, rounded and often somewhat ascending at apex, densely pustulate and coarsely ridged, dark olive-green to brown or rarely black, often pale yellowish brown in old parts. Isidia present or absent, granulose to slightly oblong, up to 0.2 mm wide. Lower surface greenish brown, pitted with a few hapters. Upper and lower cortices absent. Apothecia common in non-isidiate morphs, crowded (0.4-)0.6-1(-1.5) mm across, with a plane to convex, brownish red disc and an entire, often isidiate thalline margin. Proper exciple thin, euthyplectenchymatous; hymenium colourless, 85-110 µm tall. Asci 8-spored, clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores (4-)5-12-septate, hyaline, acicular to bacilliform, straight or spirally curved, 50-90(-110) x 3-4.5 µm. Pycnidia immersed or semi-immersed, on both sides of the thallus, pale. Conidia bacilliform or with slightly swollen ends, 4.5-6 x 1-1.8 µm. Photobiont cyanobacterial (Nostoc), in long chains. Spot tests: all negative. Chemistry: without lichen substances.
Note: a mainly temperate species found on more or less isolated trees (depending on air humidity), more common in the past, presently absent from heavily disturbed areas of the North, but locally still frequent in humid, semi-natural habitats, with optimum in Tyrrhenian Italy; the species is more wide-ranging in altitude and latitude than the closely related C. subnigrescens. The recent record from Tuscany by Pasquinelli & Puccini (2010), judging from the pictures and the description, is certainly wrong.