Description: Thallus crustose, episubstratic, very thin to very thick (0.1-1.8 mm), strongly rimose to areolate-granulose, the areoles contiguous or dispersed, flat to usually convex, white, to greyish white. Cortex overlain by a 10-20 µm thick epincecral layer; medulla white, I-. Apothecia lecideine, black, shiny at least when young, sessile and strongly constricted at base, 0.5-1.2(-1.5) mm across, with a flat to convex disc and a usually persistent proper margin (older apothecia may be immarginate). Proper exciple grey-green to brownish-green in outer part, pale brown within, with crystals dissolving in K; epithecium blackish green to bluish green, rarely olive-brown in old apothecia, 10-15 µm high, N+ red; hymenium colourless, not inspersed with oil droplets, with granules dissolving in K, 65-95 µm high; paraphyses easily made free in K, simple, rarely anastomosing or branched, c. 2 µm thick at mid-level, the apical cells up to 4 µm wide; hypothecium pale reddish brown. Asci 8-spored, clavate, with an intensely I+ blue tholus penetrated by a weakly amyloid, broadly cylindrical axial mass, and a poorly developed ocular chamber, Lecidella-type. Ascospores 1-celled, hyaline, ellipsoid, 9-17(-20) x 5.5-9 µm, with a thick, smooth wall, not halonate. Pycnidia globose, black. Conidia thread-like, strongly curved, 20-30 x c. 1 µm. Photobiont chlorococcoid. Spot tests: thallus K- to rarely K+ yellow, C- or C+ yellowish red, KC+ slowly reddish orange, P-. Chemistry: atranorin, chloroatranorin, thiophanic acid, and variable quantities of other xanthones.
Note: a widespread, chemically variable species of basic siliceous rocks, on faces wetted by rain, with a wide altitudinal range.