Lecanora praepostera auct. non Nyl.
Lecanora schistina (Nyl.) Arnold
Lecanora gangaleoides f. schistina Nyl.
Thallus crustose, areolate, (lobes and warts absent), areoles flat to slightly convex, 3-10 cm across, dull cream, cream-greyish, pale cream-greenish, often with bluish, prothallus most often present, dark-blue to blackish with light and dark zones. Apothecia usually numerous, often clustered towards the margin or the center, immersed at first then markedly sessile, 1-1.5 mm diam., disc flat, often absent and then leaving small empty cupules, pale brown, grey-brownish, sometimes pruinose, dark brown to blackish when old, sometimes with patches of different colours, exciple determinate, persistent, becoming wavy to crenulate, whitish, pale brownish. Ascospores simple, ellipsoid, 11-15 x 6-8 µm. Photobiont: Trebouxia. C-, K+ yellow quickly turning red, blood-red, Pd+ golden yellow to red-orange. On acid sheltered more or less vertical walls of rocks along rocky coasts and cliffs. Rather frequent on schists and micaschists, notably near the seashore. This species has been named Lecanora schistina by French authors. May be confused with Lecanora bandolensis that grows on less acid and well-lit rocks and with a thicker thallus, more whitish, with negative chemical spot tests and above all with apothecia discs spotted with dark green. May also be confused with Tephromela atra often growing nearby but with apothecia discs never brownish but black even when young and with a much more whitish thallus, K+ yellow. Watch out for Lecanora campestris which is K + yellow and does not have a blue black prothallus. Not to be confused with two other close species having the same chemical reactions: Lecanora gangaleoides of which the thallus is markedly verrucose and exciple determinate, rather thick and whitish and Lecanora cenisia ss. auct. brit. of which the thallus is grey-bluish and apothecia most often with brown-red discs (maybe the same or a simple morph.).
N.B. Species extraordinarily variable in particular according to the nature of its substrate; there also seem to be many taxon, see for example: Lecanora praepostera forma which has a very pale blue thallus.