Lecanora praepostera auct. non Nyl.

    Thallus crustose, not lobed, formed by small areoles a little convex, smooth to slightly warty (appearance of "lizard skin"), very flat and without small elevations or large warts, 3 to 10 cm in its largest size, dirty cream, cream-greyish, sometimes soiled with dirty brownish, often with pink tones in seawater splash areas, prothallus whitish zoned with blue-greyish if present. Apothecia generally few and scattered, sometimes grouped towards the margin of the thallus (more rarely in the center), sessile, 1-1.5 mm in diameter, flat disk, blackish or black, (rarely brown-gray in dimly lit and sunless areas), clear margin, persistent, relatively thin becoming a little flexuous or crenate, whitish. Spores simple, elliptical, 11-15 x 6-8 µm according to the literature; 13-15 x 6-7 µm according to our measurements. Numerous pycnidia clearly visible on the areoles, conidia filiform: 22-30 x 1 µm. Photobiont: Trebouxia. C-, K+ yellow then red by the presence of needle-shaped red crystals (not blood red), KC +/- orange, P+ buttercup yellow to orange-red. Species coming to the seaside on sheltered, acidic, more or less vertical rocks, in particular Armorican sandstone rocks of rocky coasts or cliffs.

N.B. This species was named Lecanora schistina by French authors.

N.B. This species can be confused with some forms of Myriolecis bandolensis with dark brown apothecia that grows on less acidic and sunnier rocks but whose thallus is thicker, whiter and whose colour reactions are negative, but especially whose disc of the apothecia is not black but brownish and spotted with dark green and also with some forms of Lecanora campestris morpho atrata with blackish apothecia that are K+ yellow and do not have a black hypothallus. But the most frequent errors of determination come from confusion with species with blackish or black apothecia and with identical or similar coloured chemical reactions such Tephromela atra sometimes coming nearby whose thallus, much more whitish, is K+ yellow and even more with Lecanora gangaleoides with a very warty thallus also K+ yellow and with apothecia with a fairly thick whitish clear margin (this last species having been formerly confused by French authors!).

N.B. Species quite variable in particular depending on the nature of its substrate, very easy to confuse with Lecanora cenisia morpho melacarpa which can share its habitat but whose thallus is more greyish-bluish and formed of larger areoles with irregular edges which are K+ yellow then blood red.