Scoliciosporum umbrinum (Ach.) Arnold
Thallus crustose, highly variable, thin to thick, smooth to rugose, uneven and sometimes nodular, more or less cracked, grey-greenish, brown-greenish, blackish, with sometimes patches of chocolate-brown. Apothecia frequent, sessile, 0.3-0.8 mm diam., disc flat and smooth, then convex and slightly rugose, brown-redish, brown-blackish, blackish, exciple usually excluded. Ascopores acicular, and spirally twisted, 3-7-septate, 20-30 x 2-3 µm. Photobiont: Chlorococcoid. Chemical spot tests negative. Probably widespread but inconspicuous and consequently understated. On acid rocks, walls, soils, wood, rust, with wide ecological amplitude. N.B. Highly variable and sometimes difficult to identify. Most probably common but most often ignored.