Opegrapha vulgata  

Thallus thin, sometimes effuse or conspicuous, smooth or ± rimose-cracked, white, grey, pale or deep brown, often with an olive tinge. Isidia and soredia absent. Photobiont Trentepohlia.
Anamorph: conidiomata pycnidia, black, often numerous, intermixed with the lirellae. Conidia of three types, sickle-shaped and 9-18 × 1-1.5 μm, or 9-14 (-17) × 1-1.5 (-1.8) μm, or bacilliform and 3-5 × 1-1.5 (-1.8) μm.
Teleomorph: ascomata lirellate apothecia, (0.5-) 0.8-2 (-3) × 0.15-0.25 mm, 50-100 µm high, very variable, sessile or semi-immersed, elongate, serpentine-sinuate, simple or often furcate, sometimes stellate or forming an interlinking network. Disc persisting as a slit, only rarely becoming partially exposed in older apothecia. Exciple K–. Epithecium dull chocolate-brown. Hymenium 45-60 μm tall, I+ red. Interascal tissue of pseudoparaphyses, septate, branched, often richly anastomosed, the apices not or rarely slightly swollen. Asci clavate-cylindrical, fissitunicate, with a minute K/I+ blue apical ring, 8-spored. Ascospores (15-) 20-30 (-40) × 2.5-4 (-4.5) μm, clavate to elongate-acicular, tapered at one end, straight or somewhat curved, 4- to 7- (to 8-)- septate, thin-walled, with a broad gelatinous sheath.
Chemistry: thallus C–, K–, KC–, Pd–, UV– (no lichen products detected by TLC).