Lecidea silacea (Ach.) Ach.

   Thallus crustose, thick, tightly attached to the substratum, cracked to areolate, areoles slightly convex, 1-1.5 mm high, surface bright, rusty, brown-red, red-orange, blood-red, prothallus absent. Apothecia usually numerous, arising between areoles, 0.5-1.5 mm diam., immersed at first, then more or less sessile, disc flat then convex, black with glossy blue-black tinge, exciple very thin, paler, soon excluded. Ascospores broadly ellipsoid, colourless, 10-15 x 4-5 µm. Chemical spot tests negative. Montane, on iron-enriched siliceous rocks, very often together with two similar species: Rhizocarpon oederi with gyrose apothecia and muriform ascospores and Tremolecia atrata with concave apothecia.