Porpidia speirea (Ach.) Krempelh.

    Thallus crustose, rather thick, surface smooth and minutely granulose, more or less cracked, chalk-white, sometimes spotted with grey or brown, prothallus usually absent or in case, blackish. Apothecia usually numerous and scattered all over the thallus, sometimes in clusters of 2 or 3, 0.5-1.5 mm diam., immersed at first, then attached to the thallus by a constricted section composed of thallus fragments, disc flat to slightly convex, sometime with a small button-like excrescence towards the center, greyish and pruinose when young, then blackish, exciple distinct, darker, rather persistent, excluded only when old. Ascii 8-spored, clavate, 100-120 x 25-35 µm according to our measures. Ascospores 12-19 x 6-7 µm according to literature, 15-22 x 7-10 µm according to our measures. Photobiont: green algae. C-, K-, KC-, Pd-, I+ dark blue-violet to dark blue. Highly variable, several forms described, montane or alpine according to French authors, ubiquist according to English authors. On slightly acid, neutral or calcareous rocks, sometimes on large rocks on the ground. N.B. Close to  Porpidia cinereoatra that is I- and restricted to acid rocks.