Distribution: N - Frl, Ven (Nascimbene & al. 2021), TAA (Nascimbene & al. 2021), Lomb, Piem (TSB 25801). S- Bas (Nascimbene & al. 2021).
Description: Thallus crustose, episubstratic, rather thick, chalky white, areolate, forming small (1-4 mm wide) patches, the areoles irregular in outline, flat to slightly convex, dispersed to contiguous and separated by deep cracks. Medulla white, I+ blue. Apothecia lecideine, black, 0.3-0.4(-0.6) mm across, subimmersed to subsessile, with a soon convex, usually epruinose disc, and a soon excluded proper margin. Proper exciple brown in outer part, paler within, c. 30 μm wide; epithecium brown; hymenium colourless, K/I+ blue; paraphyses simple, slightly capitate; hypothecium reddish brown. Asci 8-spored, clavate, Bacidia-type. Ascospores (2-)3-septate, brown, ellipsoid, slightly curved and bean-shaped, 12-19(-22) x 5-8 μm, with a minutely granulose ornamentation over the whole surface. Photobiont chlorococcoid. Spot tests: K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: an apparently widespread but rare, or at least rarely distinguished, calcicolous species, most often found on weakly to strongly calcareous rocks, often on rain-protected surfaces, mostly above treeline; it often starts the life-cycle on other crustose lichens. The species, characterised by four-celled spores with transverse septa only, and by the I+ blue reaction of the medulla, is very closely related to (and often synonymized with) the silicicolous D. lutosum, which differs in the more strongly ornamented spores and in the ecology. I have placed here all earlier records of D. lutosum from calcareous rocks in upland areas.