Rinodina luridescens (Anzi) Arnold
Quite thick crustose thallus that can reach 10-12 cm in diameter, cracked with areoles of 1.25-2.5 mm forming a mosaic, grayish, brownish-gray, typically with purplish tones if the thalli are well exposed, gray- bluish or purple, hypothallus almost always present, dark gray, blackish-brown, blackish. Apothecia numerous but quite variable, most often grouped in the center of the thallus, relatively large, 0.5 to 1.5 mm in diameter, more or less buried in the thallus (but see the note below), very thin blackish margin, little visible then excluded, black disc, flat then slightly convex. Asci with 8 spores, 50-65 x 18-25 µm. Broadly elliptical spores with a thick wall and a broad septum, 15-23 x 8-12 µm according to the literature and 17-22 x 7.5-12 µm according to our own measurements. Photobiont: Trebouxia. Cortex: K-, C+ orange-red, P-. An uncommon species, but abundant in its locations (where it can form large colonies visible from afar and characteristic for their purplish coloring), growing on siliceous rocks (especially granite) well exposed and sunny in the supralittoral zone.
N.B. Quite variable species whose purplish tinges are only very clear on dry thalli well exposed to the sun, moreover the apothecia are also very variable: sessile, brown with a lighter margin or more frequently blackish, flat and partly submerged with a barely visible margin (the two forms can be found on the same thallus!).
N.B. Pay attention in the field to certain taxa of Buellia (and in particular Buellia aethalea) which are quite similar but without purplish tones and which are C- and also to Rhizocarpon richardii which is C+ red and can have the same purplish tinges but whose apothecia are different and Rinodina atrocinerea with pale gray K+ yellow thallus..
N.B. Also pay attention to certain dark forms of Fuscidea cyathoides which is C- and P+ red and whose spores are different.