Rhizocarpon viridiatrum (Wulfen) Körber

    Thallus at first parasite, lichenicolous becoming free and crustose (but always close to its host), made of areoles, rounded to slightly angular, separate by thin cracks, 0.5-1.5 mm diam., slightly convex becoming bulging, smooth, yellow, lemon, yellow-green, faint yellow, prothallus absent, thallus spreading laterally, leaving from place to place black necrotic spots, finally gives rise to free thallus with a black prothallus, of which the development stays limited as for non-parasitic similar species. Apothecia usually numerous, rounded to sligthly angular, 0.4-1 mm diam., disc slightly convex to convex, little or not protruding, in frame with the areole surface curve, disc black, not powdery, exciple inconspicuous except sometimes when young. Ascospores brown-blackish, broadly ellipsoid, submuriform, 17-32 x 10-16 µm. Photobiont: Chlorococcoid. Medulla: C-, K+/- yellow, Pd+/- orange, I-. Rather rare in our prospection area, on weakly acid rocks, notably schists, may additionally come with its host on granite rocks. Initially parasite of crustose lichens: Aspicilia, Acarospora, Lecidea, etc. and possibly with Miriquidica deusta, often found together, but with a with a strong preference for Aspicilia caesiocinerea that it may completely  eliminate,  later, after the host destruction, may continue a free life and develop a black prothallus that makes it difficult to distinguish in the field from Rhizocarpon lecanorinum.  The latter has a more developed black prothallus, apothecia arranged towards the center of a cluster of areoles horseshoe-shaped, and larger ascospores.*