Placynthium nigrum (Huds.) Gray
Thallus crustose, thick, felt-like, wide spreading, up to 12 cm across, squamules granular or digitate, 1-2 mm, sometimes aggregated and areoles or small cushions forming when dry, blackish, brown-black, black-bluish, prothallus felted or fimbriate, carpet-fringe-like, conspicuous, typically dark blue, blue green, sometimes when fresh with a white margin. Apothecia leceidine, most often scarce and sometimes absent, 0.5-1 mm, concave at first then cup-shaped and finaly flat or convex, glossy, disc black, exciple black, persistent (except when fully mature, black and markedly convex). Ascospores colourless, narrowly ellipsoid, 1-3-septate, 9-22 x 3-6 µm. Photobiont: Cyanobacteria. Chemical spot tests negative. Not especially maritime, calcicolous, rather common on maritime works, on concrete and cement walls or on calcareous rocks, common on kersantite. Very rare on slightly acid rocks, rather frequent on ground and on compacted soils of calcareous dunes, always near stones and rocks. N.B. Many different taxa of this highly variable species have been described including some coastal. Unfortunately, the current related knowledge is very limited and they are not be presented here. Another close species (or form according to some authors), Placynthium tremniacum, grows in dryer habitats and differs by its lack of prothallus and shorter, 1-septate ascospores.