Dirina fallax De Not. forme sorédiée.

     Thallus crustose, thin, 0.5-1.5 mm thick, minutely rugose and verrucose, sometimes slightly cracked, pale grey-brown, cream to brownish with sometimes grey-purple or grey-lilac tinges, more or less dark in case of very exposed specimen, covered with large granular or subglobose soredia (sometimes suggesting isidia), grey-yellowish, scattered (and not in piles) all over the thallus, prothallus brownish, often limited and frayed. This sorediate form is sterile, but a form with pycnidia may be found : Dirina fallax forme pycnidiée, as well as chimera specimen that show simultaneously sections with abundant pycnidia and some others with soredia or even apothecia. Photobiont: Trentepohlia. Cortex : C+ red (faint and slow), K-, KC+ red, Pd-, UV- ; Medulla : C-, K-, KC-, Pd-, UV+ pale yellow. Not rare in our prospection areas, grows on sheltered vertical walls of siliceous rocks, very often near the seashore, together with Llimonaea sorediata  that differs by its pinkish farinose thallus, found also on weakly acid stones (notably schists) of sheltered walls of chapels and in that case difficult to distinguish from Dirina massiliensis f. sorediata. To the contrary, the fertile form Dirina fallax forme fertile is easier to identify but is rare in our propection area. N.B. This species is very close to Dirina massiliensis f. sorediata and can be distinguished by its thallus, thinner, its colour, brownish to ochre and above all the fact that it is restricted to acid siliceous rocks (but many intermediate specimen may be found that are not determinable on the sole macroscopic characteristics), usually associated to lichens found on acid siliceous rocks like Llimonaea sorediata and Opegrapha gyrocarpa.

More details available at: TEHLER A., ERTZ D., IRESTEDT M. The genus Dirina (Roccellaceae, Arthoniales) revisited. The Lichenologist. 45(4); 427-476 (2013).