Description: Thallus crustose, endosubstratic or thinly episubstratic, up to 0.1 mm thick, continuous to areolate, grey-white, sometimes with a yellowish tinge, more rarely greenish brown, smooth, sorediate. Soralia white, greenish or yellowish white, delimited to confluent, rounded to ellipsoid, 0.2-1 mm across, sometimes covering the whole thallus surface, flat to more rarely convex, often bordered by a thin thalline rim; soredia farinose, 20-50 μm wide. Apothecia, rare, lecanorine, 0.4-1(-1.5) mm across, at first immersed amongst the soredia, finally erumpent, with a pale reddish brown to black, finally convex disc and a distinct to finally excluded, initially sorediate thalline margin. Thalline exciple pulicaris-type, the medullary part with abundant, very large crystals which are insoluble in K; epithecium yellow- to green-brown, inspersed with granular crystals dissolving in K but not in N; hymenium colourless, 50-80(-100) μm high; paraphyses slightly branched in upper part, anastomosing in lower part, (1-)1.5-2 μm thick at mid-level, the apical cells not markedy swollen, up to 3 μm wide. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-celled, hyaline, broadly ellipsoid to subglobose, (10-)14-18(-20) x (7-)10-13(-15) μm, thick-walled (the wall 1-1.5 μm thick). Photobiont chlorococcoid. Spot tests: thallus and soralia K+ yellow, C-, KC- or KC+ faintly yellow, P- or P+ pale yellow, UV-. Chemistry: atranorin, chloroatranorin, roccellic acid, with traces of 1-2 additional fatty acids.
Note: usually on wood but occasionally also on bark; most common in NW Europe, with scattered records from humid parts of the Alps; to be looked for in the Italian Alps.
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)