Buellia subdisciformis (Leight.) Vain.
Thallus crustose, 2-5 cm diam., rather thick, cracked as an old faience, whitechalk, whitish to dull white with a pale yellow-brown tinge or sometimes green-brown, usually delimited by a clearly visible black prothallus especially in case of several colonies forming mosaics. Apothecia numerous, regularly distributed, sessile, 1-1.5 mm diam., disc plane to slightly convex, black, occasionally pruinose at least when young, exciple greyish, prominent, mostly persistent, often wavy. Pycnidia occasionally numerous and clearly visible as blackish dots. Ascospores ellipsoid, brown, 1-septate, 10-19 x 6-10 µm. Photobiont : Chlorococcales. Thallus C-, K + yellow~red (crystals), Pd+ yellow-orange. Common in xeric supralittoral zone often under overhangs of hard siliceous coastal rocks but also in the aerohaline zone on siliceous rocks and old walls when not directly exposed to rain. N.B. Variable according to the locations where it grows but always easy to recognize, particularly for atypical specimen, by the mean of chemical spot tests.*