Arthonia leucopellaea (Ach.) Almq.
Thallus crustose, very thin and partially immersed in the bark substratum, smooth to minutely rugose, whitish, white-cream, white-greyish, white-pinkish, prothallus invisible. Apothecia immersed in the thallus or very slightly prominent, highly variable in shape and distribution: more or less lobate, branched, stellate, elongate, round, etc. and distributed in small clusters or in chains or scattered, etc., brown-black, blackish most often covered with a whitish pruine giving a final grey aspect, notably towards the margin. Ascospores narrowly ellipsoid, colourless, possibly becoming brown when old, 3-4-septate without enlarged apical cell, 10-16 x 3-5 µm; (13-16 (17) x 5-6 (7) µm according to our own measures). Photobiont: Trentepohlia. Thallus: C-, K+/- yellow > dull yellow, KC-, Pd-, UV-. Rare, strong oceanic trends, on acid bark of trees near the seashore and notably on old pines (Pinus), much less frequently on bark of old oaks (Quercus). Not to be confused with Arthonia pruinata rather similar but C+ red with a more cracked thallus surface.*