Diplotomma hedinii (H. Magn.) P. Clerc & Cl. Roux

    Thallus crustose, highly variable, more or less rosette-shaped, without lobes towards the margin, 1-5 cm accross, smooth and not pulverulent, slightly cracked and forming areoles, hardly convex, most often chalkwhite, sometimes white-ochreish, pale in some sections (according to the substratum), prothallus usually absent or inconspicuous, thin and greyish at junction of different colonies. Apothecia usually numerous, 1 per areole, immersed at first then prominent often surrounded by a thalline exciple due to thallus rupture, disc flat, soon convex, black and often pruinose and whitish. Ascii 8-spored, 50-70 x 12-18 µm. Ascospores dark brown-black, more or less kidney-shaped, 3-septate, 14-21 x 6-11 µm. Photobiont: chlorococcoid. Thallus and medulla: C-, K-, KC-, P-, I-. On hard limestone together with Aspicilia calcarea. Cf. The saxicolous forms of Diplotomma alboatrum growing on calcareous rocks have negative chemical reactions but their ascospores are submuriform. In addition, Diplotomma venustum medulla is K+ yellow > red.*