Description: Thallus foliose, homoiomerous, subgelatinous when wet, dark olive-green to almost black, dull, 3-6 cm wide, rather loosely attached. Lobes 0.5-1.5 cm wide, thin when dry, 75-120 µm thick when moist, not swollen nor pustulate, more or less rotund at apex, often ascending. Upper surface with numerous simple, globose, c. 0.05-0.1 mm isidia. Lower surface usually somewhat paler than the upper surface. Upper and lower cortex absent. Apothecia rare, laminal, with constricted base, 1.5(-2.5) mm wide, with a reddish brown disc and a smooth, usually thickly pseudocorticate thalline margin. Proper exciple rather thin, subpara- to euthyplectenchymatous; hymenium colourless. Asci: 8-spored, narrowly clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores (3-)5-7-septate, hyaline, narrowly fusiform to broadly acicular, 40-55(-65) x (3-)5-7 µm. Pycnidia rare, immersed, laminal, pale brown. Conidia bacilliform or with slightly swollen ends, (3-)4-4.5(-6) x 1-1.5(-1.8) µm. Photobiont cyanobacterial (Nostoc), in long chains. Spot tests: all negative. Chemistry: without lichen substances.
Note: a mainly temperate, incompletely holarctic species found on more or less isolated broad-leaved trees in humid-rainy areas, mostly below the subalpine belt; more common in the past, presently absent from heavily disturbed habitats and most frequent in Tyrrhenian and northeastern Italy.