Buellia stellulata  

Thallus crustose, thin, rimose, white to pale grey, rimose-cracked, mostly 10-20 mm diam., delimited by a conspicuous black prothallus and often forming mosaics. Areoles 200-500 µm wide, flat, epinecral layer present. Medulla I–. Photobiont chlorococcoid.
Anamorph: conidiomata pycnidia, immersed in the thallus, rarely seen, the wall dark brown at least in the upper part. Conidia 3.5-5 × 1-1.5 μm, cylindrical to bacilliform, hyaline, aseptate.
Teleomorph: ascomata apothecia, 200-350 µm diam., immersed, dark grey to black. True exciple thin, evident at least when young. Epithecium dark olive to olive-brown, N+ red. Hymenium hyaline, I+ blue, without oil droplets. Hypothecium dark brown. Interascal tissue of paraphyses, septate, simple or branched in the upper part, the apices swollen and pigmented, with a dark brown cap. Asci clavate, 8-spored. Ascospores (8-) 10-13 (-15) × 5-7 (-8.5) μm, the wall evenly thickened, dark brown when mature, not constricted at the ± median septum, smooth or minutely roughened, without an epispore, gelatinous sheath or appendages.
Chemistry: thallus C–, K+ yellow, Pd± yellow, UV– (atranorin, confluentic and 2′-O-methylperlatolic acids).