Lecidea fuscoatra (L.) Ach.
Thallus crustose, to 10 cm across, markedly areolate, areoles thick, 2-3 mm across, flat to convex, extending to thallus margin, glossy, brown-reddish, pale brown, fawn, grey-brown, prothallus black. Apothecia numerous, 1-2.5 mm diam., innate then sometimes a little sessile, disc flat to convex, black but often grey owing to dense pruine, exciple thin, concolourous, soon excluded. Ascospores ellipsoid, colourless, simple, 11-16 x 4,5-6,5 µm. Photobiont: Trebouxia. Cortex: C+ red, K-, KC+/- red, Pd-. Slightly nitrophile, on siliceous rocks, common in Central Brittany and to a lesser extent on rocks near the seashore. N.B. Lecidea grisella Flörke is a sympatrical species in Brittany and notably in coastal areas. Its thallus is more rimose, less areolate except when old and towards the center. When existing, areoles are gradually thinner near margins, colour is more greyish and matt. Both species may grow together and sometimes are difficult to distinguish. When a black prothallus is present and well developed and the thallus overall colour very dark, see also Miriquidica deusta, most often sterile. N.B. Also watch out for Immersaria athroocarpa very similar but whose apothecia are immersed in the thallus with a concave disc and which is C-.