Arthonia atra  

Thallus thin, smooth, effuse or delimited, cream- to silver-white, sometimes tinged olivaceous, smooth, often immersed, usually in small, clearly defined patches, occasionally mosaic-forming. Prothallus ± distinct, grey to black. Isidia and soralia absent. Photobiont Trentepohlia.
Anamorph: conidiomata pycnidia, immersed to erumpent, rather irregular in shape. Conidiogenous cells in a palisade lining the inner wall, cylindrical, proliferatin percurrently with a very long ± cylindrical collarette. Conidia 4-5 × 1-1.5 μm, bacilliform, straight or slightly curved, the ends blunt and rounded, aseptate, hyaline, thin-walled, without an epispore or gelatinous sheath.
Teleomorph: ascomata apothecia, (300-) 700-1500 (-2000) x (90-) 120-200 (-300) µm, 100-130 µm high, abundant, randomly distributed or in parallel lines or stellate clusters, variable, simple or frequently branched, sometimes forming dense, contiguous, black patches, sessile. Disc a slit, rarely opening. Thalline exciple absent, true exciple black, not pruinose, continuous under the hypothecium, K+ olive-green. Hymenium 60-90 μm tall, I+ blue. Epithecium brown or green-brown, K+ olive-green. Hypothecium ± hyaline. Interascal tissue of pseudoparaphyses, septate, branched, often richly anastomosed, the apices not or rarely slightly swollen. Asci clavate-cylindrical, thick-walled and fissitunicate, with a minute K/I+ blue apical ring. 8-spored. Ascospores 13-18 (-20) × 2.5-4 (-5) μm, ellipsoidal or often ± clavate, 3-septate, the median cells not enlarged, hyaline to pale yellow, smooth, with a thin gelatinous sheath.
Chemistry: thallus C–, K–, KC–, Pd–, UV– (no lichen products detected by TLC).