Rhizocarpon petraeum (Wulfen) A. Massal

    Thallus crustose, most often rounded, to 5 cm diam., or more, cracked and often slightly areolate towards the center, areoles angular, 0.5 mm across, crowded, white-chalk, whitish, pale grey, prothallus thin and narrow, dark grey. Apothecia up to 0.5 mm diam., sometimes arranged in concentric circles, innate, often angular or oval, disc flat becoming convex, rough, black, not pruinose, exciple persistent, paler and tinged with blue or green, sometimes white owing to pruine of the same colour. Ascii 8-spored, 120-140 x 30-50 µm according to our measures, Ascospores muriform, at first colourless, then  blackish, 20-50 x 13-25 µm according to literature, 22-37 x 12-20 µm according to our measures. Photobiont: Chlorococcoid. Medulla : K+ yellow, Pd+ orange. Not especially maritime, but not so rare on walls near the seashore when alkalised by the mortar, or on neutral rocks. Not to be confused with Rhizocarpon reductum of which the thallus usually is smaller and more cracked, less whitish or darker and with a conspicuous well-developed black prothallus, growing on siliceous rocks. See also some resembling Buellia species of which the ascospores are not muriform.