Description: Thallus foliose, homoiomerous, subgelatinous when wet, membrane-like, rounded, closely adnate, broadly lobed, 1.5-3(-4) cm wide,. Lobes 0.5-0.8(-1) cm wide, 62-160 µm thick when 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 white hapters. Upper and lower cortices absent. Apothecia common, lecanorine, sessile, constricted at base, 0.4-1(-1.5) mm across, with an initially concave, finally convex, red-brown to almost black disc and a thin, smooth thalline margin. Thalline exciple sometimes partly pseudocorticate; proper exciple euthyplectenchymatous, 20-40 µm wide laterally; epithecium brownish; hymenium colourless, 65-85 µm high, I+ blue; paraphyses simple or sparingly branched in upper part, 2.5-3 μm thick at mid-level, the apical cells moderately 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 3-septate, not constricted at septa, hyaline, bacilliform with rounded ends, straight or slightly 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, the cells 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).