Buellia subdisciformis  

Thallus thin to thick, rimose-cracked, areoles 0.2-1.2 mm diam., sometimes becoming warted, pale to medium-grey, sometimes with a yellow tinge, usually delimited by a black prothallus, often mosaic-forming; medulla I–. Isidia and soralia absent. Photobiont chlorococcoid.
Anamorph: conidiomata pycnidia, numerous. Conidia (6-) 9-13 × 0.8-1 μm, cylindrical, not curved.
Teleomorph: ascomata apothecia, 0.4-1 (-1.8) mm diam., superficial. Disc plane to slightly convex, disc and margin grey-white pruinose at least when young. Exciple prominent, mostly persistent, uniformly pigmented throughout. Epithecium brown, N–. Hymenium without oil droplets, hyaline to pale yellow. Hypothecium dark red-brown, very well-developed. Interascal tissue of narrow thin-walled paraphyses, septate, unbranched or branched in the upper third, the apices swollen and pigmented, with a dark brown cap. Asci clavate, Lecanora type, 8-spored. Ascospores 9.5-19.5 × 5.5-10 μm, ellipsoidal, the apices rounded, 1-septate, the wall uniformly thickened, finely warted, without an epispore, gelatinous sheath or appendages.
Chemistry: thallus C–, K+ yellow→red (crystals), Pd+ yellow-orange, UV– (atranorin, norstictic acid, traces of other substances of the stictic acid complex).