Buellia aethalea  

Thallus thin to rather thick, distinctly areolate, often less than 2 cm diam. and mosaic-forming. Areoles 0.2-0.6 (-1.2) mm diam., ± angular, flat or rarely convex, pale to dark grey, sometimes discoloured brown, delimited by a narrow, black prothallus that is sometimes visible between the areoles. Medulla I± blue. Photobiont chlorococcoid.
Anamorph: conidiomata pycnidia, rarely present. Conidia 5-7 (-8.5) × ca 1 μm, bacilliform.
Teleomorph: ascomata apothecia, 0.2-0.4 (-0.7) mm diam., immersed, sometimes surrounded by a thalline collar. Disc flat. True exciple thin or inconspicuous, usually dark olive, distinctly pigmented only in the outermost part. Epithecium brown to olive, N± red. Hypothecium pale to dark brown. Interascal tissue of paraphyses, septate, branched in the upper third, the apices swollen and pigmented. Asci clavate, Lecanora-type, 8-spored). Ascospores (9.5-) 12-18 (-20) × (4.5-) 6-10 (-12) μm, 1-septate, initially blue-green but eventually becoming dark brown, thick-walled, minutely warted, without a distinct epispore, gelatinous sheath or appendages.
Chemistry: thallus C–, K± yellow→red (crystals), Pd± yellow-orange, UV– (usually norstictic, and ± stictic acid; sometimes no lichen substances detected by TLC).