Racodium rupestre  Pers.

    Thallus filamentose, forming cottony or woolly mats, 10-15 cm across, 0.3-0.5 mm tall, sometimes continuous but most often discontinuous in small tufts, composed of filaments, 10-15 µm diam., and 0.2-1 cm long, in which the photobiont (Trentepohlia) is thread-like and surrounded by a parallel hyphae, delimiting rectangular spaces (like a brick wall), brown-blackish when moist and blackish when dry. Fertile form unknown. Chemical spot tests negative. Common, everywhere on vertical walls or in overhangs of humid siliceous rocks but not directly subjected to precipitations. Very difficult to distinguish from Cystocoleus ebeneus without microscope observation (hyphae organized in network and not parallel), but the latter is rarrer and restricted to heights on schist ridges (where it may grow together with Racodium rupestre). N.B. It is quite common to found similar black zones on rocks and humid walls in runoffs, but most often they are the result of filamentous cyanobacteria development (mainly belonging to the Stigonema genus) and not of lichens. On the field, these cyanolichens give rise to a thin felt mat, 2-3 mm thick, not cottony nor woolly and more or less gelatinous when moist.*