Dirina fallax De Not. f. fallax
Dirina repanda v. schistina Bagl.
Thallus crustose, rather thin, 0.2-1.5 mm thick, more or less smooth, cracked, often rimose to verrucose, colours highly variable according to substratum and exposure: grey-brownish, greyish, ochre often tinged brown-purple or grey-purple in areas subjected to seaspray, prothallus very thin and frayed, seldom zonate, pale brown-ochre. Apothecia often numerous (see Dirina fallax f. fallax forma), scattered mainly towards the thallus center, immersed at first, becoming warted and finally sessile, constricted towards the base, 0.3-1.5 mm diam., exciple uneven, slightly wavy, concolourous to the thallus, disc greyish to dark brown, pruinose. Ascospores colourless, ellipsoid, 3-septate or more, 18-24 x 5-6 µm. Photobiont: Trentepohlia. Thallus: K-, C+ faint and fleeting red, KC+ red, Pd-; disc of apothecia : C+ red; medulla : C-. This fertile form is very rare, grows on vertical walls or below overhangs of acid siliceous rocks. The sorediate morph, Dirina fallax f. sorediata is much more common (sometimes chimera may be observed with on the same thallus sorediate sections and fertile sections: see pictures).
More details are available at: TEHLER A., ERTZ D., IRESTEDT M. The genus Dirina (Roccellaceae, Arthoniales) revisited. The Lichenologist. 45(4); 427-476 (2013).