Description: Thallus foliose, homoiomerous, subgelatinous when wet, 1.5-3(-4) cm wide, membrane-like, rounded, closely adnate, broadly lobed. Lobes 0.5-0.8(-1) cm wide, 62-160 µm thick (moist), rounded, with an entire, not swollen magin, dark olive-green to black, lighter and semi-translucent when moist, strongly radiately ridged and pustulate. Lower surface paler green than upper surface, at times bluish green, with depressions corresponding to ridges, and scattered hapters. Upper and lower cortices absent. Apothecia common, lecanorine, sessile with a constricted base, 0.4-1(-1.5) mm across, the disc at first concave, finally convex, red-brown to almost black. Proper exciple euthyplectenchymatous, 20-40 µm wide; epithecium brownish; hymenium colourless, 65-85 µm high; paraphyses simple or sparingly brnches in upper part, 2.5-3 μm thick at mid-level, the apical cells moderately swollen. Asci 8-spored, clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores 3-septate, not constricted at septa, hyaline, bacilliform with rounded ends, straight or curved, (18-)26-34(-38) x 3-4.5 µm. Pycnidia often numerous, immersed to semi-immersed, on both sides of thallus. Conidia straight, bacilliform or with slightly swollen ends, 4.5-5 x c. 1 µm. Photobiont cyanobacterial (Nostoc), in long chains. Spot tests: all negative. Chemistry: without lichen substances.
Note: a mild-temperate to boreal lichen found on bark of broad-leaved trees in humid but open situations; easily mistaken with C. nigrescens or C. subnigrescens, this exceedingly rare species needs further study. It is included in the Italian red list of epiphytic lichens as “Critically Endangered” (Nascimbene & al. 2013c).