Buellia saxorum A. Massal
Thallus crustose, often mosaic-forming, moderately thick, rimose-crackled, dull white, greyish, pale yellow-grey, grey-greenish, grey-glaucous (the more sheltered the darker), delimited by a dark-grey prothallus. Apothecia superficial more or less sessile, 0.3-1 mm diam., disc flat to slightly convex, blackish; true exciple thick, prominent, rather persistent. Ascospores broadly ellipsoid, 1-septate, smooth, wall uniformly thickened, 10-18 x 5-9 µm. Pycnidia very numerous drawing like dotlines on thallus (see the form only with pycnidia: (see the form only with pycnidia: Buellia saxorum forme pycnidiée ). Photobiont : Chlorococcales. Medullae: I+ blue, thallus C + red-orange, pink, K + dirty yellow sometimes with traces of red or orange, KC +/- red, CK+ red, P +/- yellow. Not especially maritime but found on exposed, hard siliceous rocks, stones of old walls notably chapels (in this case often on Armorican sandstone). N.B. Similar to Rhizocarpon by its superficial apothecia. Sea also Buellia subdisciformis which thallus is whiter and chemical spot tests different. The prothallus, grey or blackish clearly delimiting mosaics, characterizes this rather little known species and deserves attention.
N.B. It seems that there are several taxa (chemoforms with variable K reaction?) of this rather variable species, in particular forms whose thallus is colored yellowish, greenish, celadon, pinkish, etc. but still pale and pastel colors (see photographs). These taxa can come close to the more whitish and pale grayish ones.