Description: Thallus foliose, homoiomerous, gelatinous when wet, blackish brown to black, thick, smooth, loosely attached,, to 6 cm diam. (usually less). Lobes (0.5-)1-2(-3) cm broad, rounded, flattened, contiguous, adpressed, with an entire, undulated, not swollen margin. Isidia globular when young, but soon becoming squamiform, diffuse, mostly simple. Upper and lower cortex absent. Apothecia rare, lecanorine, sessile, to 2.5 mm across, with a reddish brown, flat to finally convex disc, and a thin, entire, pseudocorticate thalline margin. Epithecium brownish; hymenium colourless, I+ blue; paraphyses simple or sparingly branched, 2(-4) µm thick, with clavate to subglobose tips; hypothecium colourless. Asci 8-spored, narrowly clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores transversely 3-5-septate, hyaline, fusiform-elongate, (20-)25-35(-40) x 6-7 µm. Pycnidia common, usually thicker than thallus. Conidia bacilliform, straight, (3-)4-4.5(-6) x 1-1.5(-2) µm. Photobiont cyanobacterial (Nostoc), in long chains. Spot tests: all negative. Chemistry: without lichen substances.
Note: a mainly temperate to southern boreal-montane lichen with a fragmented holarctic range, found on bark, epilithic mosses, base-rich siliceous and slightly calciferous rocks in sheltered, humid situations; more common in the past and now absent from urban areas and from the Po-Plain; mostly Tyrrhenian in Italy, but still locally frequent in humid Alpine and pre-Alpine areas.