Description: Thallus foliose, homoiomerous, gelatinous and 140-300 µm thick when wet, olive-black to black, forming irregular, deeply lobed, 3-6 cm (rarely more) wide rosettes. Lobes (2-)3-4(-5) mm broad, concave, smooth, ascending, with a coarsely undulate, not swollen margin, lacking isidia or with granulose, 0.1-0.2 mm wide isidia; lower surface paler, with scattered to confluent tufts of white hapters. Upper and lower cortices absent. Apothecia common (rarer in isidiate specimens), lecanorine, sessile, (0.7-)1-1.5 mm across, with a flat to convex, red-brown to rarely black-brown disc and a thin, entire to crenulate thalline margin. Thalline exciple ecorticate or pseudocorticate in basal parts; proper exciple euparaplectenchymatous, up to 130 µm wide laterally; hymenium colourless, 85-105 µm high, I+ blue; paraphyses simple or sparingly branched in upper part, 2-3(-4) µm thick at mid-level, the apical cells clavate, 4.5-6.5 µm wide; hypothecium pale yellow to yellowish brown, I+ blue. Asci 8-spored, clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores 3-septate, not or only slightly constricted at septa, hyaline, linear-oblong with rounded ends, 17-30 x 6.5-9 µm. Pycnidia common, mostly marginal, paler than thallus. Conidia bacilliform, 4-5 x 1.1.5 µm. Photobiont cyanobacterial (Nostoc, the cells in long chains). Spot tests: all negative. Chemistry: without lichen substances.
Note: a temperate to arctic-alpine, probably circumpolar lichen found on calciferous rocks with some water seepage after rain, most frequent in upland areas. The record from Venezia Giulia cited by Nimis (1993: 264), being from outside Italy, is not accepted here.