Description: Thallus crustose, episubstratic, white to pale grey, continuous to verrucose-areolate, very thick, forming up to 4 cm wide, orbicular patches. Cortex colourless, indistinctly paraplectenchymatous; medulla chalky white, I-. Apothecia lecideine, at first immersed then subsessile, 0.4-1.5 mm across, with an initially flat, then soon strongly convex, often slightly white-pruinose disc, a thin, soon excluded proper margin, and usually a pseudothalline margin in young apothecia. Proper exciple dark brown in section, not continuous with hypothecium; epithecium brown; hymenium colourless, K/I+ blue; paraphyses simple, 1-1.5 μm thick at mid-level, the apical cells 2.5-4.5 μm wide; hypothecium pale brown. Asci 8-spored, clavate to cylindrical-clavate, the apical dome K/I+ dark blue with a pale, conical-pointed apical cushion (axial mass), the wall I-, but the thin outer gel I+ blue, approaching the Bacidia-type. Ascospores 3-septate, brown, ellipsoid, often slightly curved, 16-22 x 7-10 μm. Photobiont chlorococcoid. Spot tests: medulla K+ yellow turning red (needle-like crystals), C-, KC-, P+ yellow, UV-. Chemistry: medulla with norstictic acid.
Note: this mild-temperate to Mediterranean lichen, at least when young, is a constant parasite on Protoparmeliopsis versicolor, reaching above treeline south of the Alps. Morphological and biological differences towards D. hedinii are so evident that the synonymisation of the two taxa proposed by Nordin (1996), based on samples from southern Sweden (!) and on chemical characters only, is not accepted here. See also note on D. hedinii and Roux & Gueidan (2002: 145-146).