Rhizocarpon oederi (Weber) Körb.
Thallus crustose, 2-5 cmm diam., margin not lobate and prothallus evanescent, cracked and areolate, areeoles angular, 0.4-0.8 mm across, rusty, reddish, red-orange, ochre-orange. Apothecia numerous, irregular, 0.3-0.5 mm diam., disc black, not powdery, concave to plane, umbonate or gyrose, exciple black, gyrose. Ascospores broadly ellipsoid, colourless, 3-septate, 12-18 x 3-7 µm. Photobiont: Chlorococcoid. Chemical spot tests negative. This species is characteristic of siliceous rocks enriched in iron or other metals, notably in ancient mines tailings, in quarries and slate quarries where they may develop large colonies, often together with Placopsis lambii. This species has montane trends, and, in our prospection area, seems to be restricted to the center of Finistère, and to be absent from potentially favourable sites near the seashore. Worth knowing that this species may grow together with a very similar species, Tremolecia atrata of which apothecia are concave, not gyrose and ascospores simple. In mountain, not to be confused with a very similar species, Lecidea silacea of which apothecia are convex, not gyrose, and ascospores simple.