Buellia aethalea (Ach.) Th. Fr.
Thallus crustose, up to 2-3 cm diam., mosaic-forming, highly variable, ± areolate, areoles angular, flat, pale to dark grey, sometimes discoloured brown; delimited by a narrow, black prothallus, sometimes visible between the areoles. Apothecia 0.2-0.5 mm diam., mostly abundant in the thallus center, immersed, rarely surrounded by a thalline thin and irregular collar (if all the apothecia display a thick margin, Rhizocarpon reductum can be suspected); disc flat, usually dark olive. Spores brown, broadly ellipsoid, 1-septate, with a slight constriction at the septum for very ripe spores, 13-18 x 6-10 µm according to the literature, (13) 14-17 (18) x (5) 6-10 µm according to our measurements. Thallus C-, K+/- yellow then red, P+/- yellow-orange. Photobiont : Chlorococcales. Not especially maritime, common on siliceous rocks in nutrient-rich situations, notably harbours, seashore parkings where they can develop large colonies, often associated with pioneer species like Rhizocarpon reductum with better formed apothecia and different spores and Buellia ocellata with yellowish thallus and C + orange. N.B. On the seashore, this species is often confused with Buellia stellulata of which the thallus may grow on the same substratum but paler to almost whitish and K+ yellow. In highly exposed zones of the seashore, not to be confused with Rhizocarpon richardii of which the thallus presents purple tinges and is faint C+ red.
N.B. This species is very variable and there are probably several taxa described under this name, see Buellia aethalea forme sur substrats artificiels.