Lepraria incana (L.) Ach. s.l.
Thallus leprose, farinose, composed of minute globose soredia, 0.05-0.1 mm diam., sometimes covered with more or less short spiking hyphae (only visible under microscope or magnifying lens), crust-forming, crust more or less thick, occasionally areolate or fractionate, margin diffuse, grey-greenish tinged blue, grey-bluish, green-yellowish tinged blue, sometimes with orange or purple-brown spots. Apothecia unknown. Photobiont: green algae. Thallus: C-, K+/- yellowish, KC-, Pd-, UV : UV: white-bluish but sometimes pale orange (worth to mention: one chemotype is K+ purple and KC+ purple). Species taken here in the broad sense because there are probably several taxa under this name; very common, always in shaded areas and protected from direct rainfall, in fissures and crevices of rocks, stones of old walls, on north surfaces of tree trunks in crevices of rough and mossy barks.
N. B. As for most of the Lepraria species it is necessary to carry out a chemical analysis to confirm the determinations, the photos below are only based on the habitat, the bluish tones of the specimens, the chemical spot tests and the observation of the thallus under the microscope, but these characters may suffice if we consider this species in the broad sense. Moreover, chemical analysis is sometimes insufficient, on its own, because there are many chemoforms that are difficult to interpret and to complicate the problem, sequencing is not possible with reference herbarium samples that are too old! (it is necessary to analyze very fresh specimens).
There is a characteristic form of old slate quarries: Lepraria incana forme saxicole des parois verticales it can cover large areas on wet and shaded walls in excavations.
N.B. There is a very similar but much rarer species in the crevices of the siliceous rocks: Lithocalla ecorticata which is K+/- yellow and KC +/- yellow. The thallus has yellow colors and its underside is whitish.