Opegrapha lithyrga Ach.
Thallus crustose membranous, very thin, smooth, gray-greenish, glaucous gray, greenish, sometimes, but rarely, rusty-brown, hypothallus not very clear, paler. Apothecia numerous, in lirelles, 0.5-2 x 0.1-0.3 mm, sessile or as if placed on the thallus, sinuous and serpentine, simple to rarely forked or star-shaped, black, disc barely visible at the bottom of the a slit. Spores very elongated, 5-7 septate, 20-35 x 3-5 µm. C-, K-, KC-, P-. Rare species occurring on the vertical walls of siliceous rocks smooth, humid and always sheltered from the light of coastal cliffs or ravines in oceanic forests. N.B. Strongly resembles Opegrapha vulgata (to the point that some authors make it a synonym) but this last species is corticole. N.B. If the apothecia are immersed in the thallus, see a fairly similar species that comes in the same environments: Enterographa hutchinsiae..