Description: Thallus crustose, endolithic or (rarely) very thinly epilithic, white to grey, sometimes with a violet hue, often delimited by a dark prothallus. Apothecia 0.2-0.8(-1) mm across, often slightly immersed in pits in the rock, with a flat to slightly convex, not pruinose, black disc and a thin, finally excluded proper margin. Proper exciple brown in outer part, colourless to brownish within, the pigmented parts K+ violet; epithecium grey to brownish (in thick sections), K+ violet; hymenium colourless, with small crystals; apical cells of paraphyses 2-5 μm wide; hypothecium colourless. Asci 8-spored, clavate to broadly clavate, Teloschistes type. Ascospores 1-septate, polarilocular, hyaline, ellipsoid, (11-)15-18(-21) x (6-)7-8(-11) μm, the septum 2.5-3 μm thick. Photobiont chlorococcoid. Spot tests: thallus K-, C-, KC-, P-; pigmented parts of apothecia K+ violet and sometimes N+ pale violet. Chemistry: no lichen substances in thallus, Sedifolia-grey pigment in apothecia.
Note: on hard limestones and dolomite on sunny, exposed cliffs, with a wide altitudinal range; in Italy this is one of the most frequent species of Pyrenodesmia. For further details see Muggia & al. (2007).