Buellia stellulata (Taylor) Mudd
Thallus thin, mostly 1-2 cm diam., rimose-cracked, areoles 0.2-0.5 mm wide, flat, smooth, seldom whitish but often bright pale metal-blue, delimited by a black radiating prothallus sometimes very large and often forming mosaics, producing a whole assemblage rather dark. Apothecia 0.2-0.35 mm diam., immersed at first then sligthly prominent, disc dark-brown and black when old; exciple thin, black, disappearing rapidly. Ascospores broadly ellipsoid, minutely roughened, 1-septate, 10-13 x 5-8 µm. Photobiont : Chlorococcales. Thallus: C-, K+ yellow, Pd +/- yellow. On hard, smooth siliceous, rocks and pebbles, mainly coastal in the xeric supralittoral zone, but see a particular habitat: Buellia stellulata forme sur substrats artificiels. Often confused, in spite of its large blackish prothallus and its small size, with Buellia ocellata ou Buellia aethalea. Identification requires chemical spot tests and microscope observations. May also be confused with Rhizocarpon reductum which chemical spot tests are negative and thallus grey-brown (ascospores are discriminant !) and with Rhizocarpon richardii on rocks; but the latter has bigger thallus with violaceous glints and different apothecia.