Description: Thallus crustose, continuous or rimose, yellowish white to pale grey, sometimes delimited by a bluish black prothallus. Apothecia lecanorine, mostly sessile, 0.4-1(-1.5) mm across, with a brown to blackish brown, flat to slightly convex disc, and a usually smooth, persistent thalline margin. Thalline exciple corticate, the cortex very thick, 20-40 µm laterally, 35-65(-90) mm thick at base, the medulla with numerous algal cells, and with large crystals insoluble in K; proper exciple colourless, lacking crystals; epithecium brown to dark brown, the pigment dissolving in K, , with small crystals (<1 μm wide) soluble in K but not in N, penetrating into the epithecium for c. 20 μm, pulicaris-type; hymenium colourless, not inspersed with oil droplets; paraphyses slightly thickened at apex; hypothecium colourless. Asci 8-spored, clavate, very thin-walled, with a K/I+ blue, tall tholus penetrated by a faintly amyloid apical cushion, the wall K/I-, surrounded by a blue outer layer, Lecanora-type. Ascospores 1-celled, hyaline, broadly ellipsoid, 11-18 x 7-12 µm, the wall 1-1.2 µm thick. Photobiont chlorococcoid. Spot tests: K+ yellow, C-, KC-, P- or P+ pale yellow. Chemistry: atranorin (major), chloratranorin, and roccellic acid.
Note: a circumboreal-montane lichen found on acid bark, often on twigs, sometimes on lignum, mostly in upland areas, with optimum in the subalpine belt; common in the Alps, becoming rarer towards the south but reaching the mountains of Calabria.