Opegrapha vermicellifera  

Thallus thin, smooth, continuous or finely rimose-cracked, often forming extensive, pale grey or medium grey patches.
Anamorph: conidiomata pycnidia, prominent, (100-) 150-250 µm diam. and 150-200 (-400) µm high, white- to pale grey-pruinose; young pycnidia semi-immersed, chestnut-brown with a pale ostiole. Conidia (3-) 4-7 × 1-1.5 μm, bacilliform, straight, hyaline, aseptate, thin-walled, without an epispore or gelatinous sheath.
Teleomorph: ascomata lirellate apothecia, (0.3-) 0.6-1.2 (-1.6) × 0.09-0.15 (-0.25) mm, 0.2-0.5 mm high, rather rare, often confined to restricted areas on the thallus, elongate, narrow, semi-immersed, straight to serpentine, simple, infrequently furcate or in scattered, stellate clusters. Disc slit-like, occasionally partially exposed. Exciple sometimes furrowed or grey-pruinose, K–. Epithecium brown. Hymenium 50-60 μm tall, I–. Ascospores 16-25(-30) × 3-4 μm, 4- to 7-septate, with rather pointed ends, hyaline, smooth, without an epispore or gelatinous sheath.
Chemistry: thallus and pycnidia C–, K–, KC–, Pd–, UV– (no lichen products detected by TLC).