Description: Thallus crustose, episubstratic, rimose or consisting of scattered to more rarely contiguous areoles, chalky white, yellowish white or pale yellowish green, often delimited by a dark prothallus, the areoles mostly flat, measuring 0.6-1.8 x 0.2-0.8 mm. Apothecia lecanorine, 0.6-2.2 mm across, with a grey-black to dark bluish green disc (sometimes grey-brown when young) and a thick, raised, persistent thalline margin. Thalline exciple 40-80 µm wide laterally, poorly differentiated from the 50-100 µm wide proper exciple, their structure masked by numerous small crystals; epithecium medium brown but often blue-green-spotted, with an epipsamma of very thin crystals, the bluish parts N+ purple; hymenium colourless, 40-50 µm high; paraphyses rather lax, branched in upper part, the apical cells swollen, 3-6 µm wide; hypothecium and subhymenium colourless, the hypothecium with large crystals visible under polarized light. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-celled, hyaline, ellipsoid, 6-9 x 3-5 µm. Photobiont chlorococcoid. Spot tests: thallus K+ faintly yellow, C-, KC+ yellow; apothecial margin P+ yellow-orange. Chemistry: thallus with usnic acid, apothecial margin with psoromic acid.
Note: on schists poor in calcium carbonate, usually on south-exposed steep faces in the alpine belt; known from the Pyrenees and from a few localities of the Western Alps (France); to be looked for in the Italian Alps.