Arthonia pruinata (Pers.) Steud. ex A. L. Sm.
Thallus crustose, rather thick, markedly cracked, soredia absent, pale greyish, pale pearl-grey, grey-whitish, pruinose, pruine pale grey-bluish disappearing when eroded or/and washed, making the underlying white thallus visible. Apothecia numerous (but hardly visible below the dense whitish pruine, except when washed out), highly variable in shape, round, angular, shortly branched or stellate, discs concave to flat, brown-pink, pale brown-greyish, very pale brown, usually veiled by a dense pruine. Ascospores oblong, colourless, 4-5-septate, 13-22 x 5-7 µm. Pycnidia common, immersed in the thallus, pale brown. Photobiont: Trentepohlia. C+ red, KC+ red, K+ pale yellow, Pd-. This species grows on dry barks of deciduous trees, notably Quercus and may cover large surfaces. See also Schismatomma decolorans of which the thallus is browner, less cracked, sorediate and C- and Arthonia leucopellaea of which the thallus is thinner, not cracked and C-, K+ yellow.