Rhizocarpon lecanorinum Anders

    Thallus crustose, up to 5-6 cm across or more in case of mosaic formation by coalescence of colonies, areolate, areoles 1-2 mm diam., smooth, matt, sometimes flat but most often convex to rounded (and in that case sometimes isolate or in very small cluster), partially or totally enclosing apothecia but typically horseshoe-shaped, yellow, pale yellow-lemon, bright yellow, yellow-greenish, prothallus usually well developed, black. Apothecia 0.5-1 mm diam., one per areole (very seldom several), more or less circular, rather flat, disc black and not pruinose, exciple thin , sometimes less distinct at the end. Ascospores brown-blackish, muriform with 15-38 cells in section, 34-57 x 15-24 µm. Photobiont: Chlorococcoid. Medulla : C-, K+ yellow, Pd+ orange. Species with mountane or colline trends, quite rare in our prospection area, coming on exposed siliceous rocks. Sometimes very difficult to distinguish in the field from some much more common Rhizocarpon geographicum morphotypes. Be careful also in case of yellow-greenish to lime colorations with Rhizocarpon viridiatrum whose spores are much smaller and which is a parasite of Aspicilia caesiocinerea.