Protoparmeliopsis muralis (Schreb.) Choisy var. muralis

    Thallus crustose, tightly appressed, more or less regular rosette-forming, to 10 cm diam., composed of small lobes, 1-2 mm wide, distinctly visible towards margins, packed without black separation lines, pale grey-greenish, pale yellow-greenish, progressively degraded from the center, becoming brownish or blackish before being excluded. Apothecia numerous, sessile, sometimes crowded towards the thallus center and then distorted and angular, 0.5-2 mm diam., disc yellow-ochraceous, pale brownish, exciple crenulate, whitish. Ascospores simple, ellipsoid, 9-15 x 5-7 µm. Photobiont: Trebouxia. C-, K-, KC+ pale yellow, Pd+ hardly visible faint yellow. Nitrophile and not especially maritime. The type prefers nutrient-rich substrata like old walls, concrete, cement, rocks, various types of grounds like pavements in harbours (and in that case hardly identifiable due to erosion and suggesting chewing-gum patches).

N.B. This highly nitrophilous and pollution-tolerant species is extraordinarily variab it would be most appropriate to consider a Protoparmeliopsis (Lecanora) muralis group since it probably encompasses several taxa, chemotypes not being systematically correlated to variants defined by their morphological traits. 

N.B. On the Monts d'Arrée ridges, a calcifuge saxicolous and not nitrophile taxon, Protoparmeliopsis (Lecanora) muralis var. dubyi, may be found, forming small cushions, margins highly foliose, preferentially on poor substrata.