Distribution: N - VG, Ven (Lazzarin 2000b), TAA (Nascimbene & al. 2007b), Lomb (Arosio & al. 2003), Piem (Arosio & al. 1998, Bari & al. 2000), VA (Matteucci & al. 2008, Isocrono & al. 2008), Emil (Fariselli & al. 2020), Lig (Giordani & Incerti 2008). C - Tosc (Loppi & Frati 2006), Marc (Nimis & Tretiach 1999), Umb (Ravera 1998, Panfili 2000b, Ravera & al. 2006), Laz (Ravera 2001, Ruisi & al. 2005, Brackel 2015), Abr (Nimis & Tretiach 1999, Brackel 2015), Mol (Nimis & Tretiach 1999, Caporale & al. 2008), Sar (Zedda 2002). S - Camp (Aprile & al. 2003b), Pugl, Bas (Nimis & Tretiach 1999), Cal (Puntillo 2011), Si.
Description: Thallus crustose, whitish to pale grey, thin, continuous to rarely cracked-areolate, effuse, sometimes almost disappearing, containing numerous clusters of coarse crystals visible under polarized light. Apothecia lecanorine, round, dispersed to crowded, sessile, constricted at base, 0.3-0.7(-1.3) mm across, with a pale brown to brown-black, flat to finally convex, sometimes white- or grey-pruinose disc, and a thin, whitish, sometimes finally almost excluded thalline margin. Proper exciple poorly developed, paraplectenchymatous, of conglutinated cells; epithecium pale brown, olive-brown or violet-brown, sometimes with granules on the surface, N-; hymenium colourless, (45-)65-70 µm high; paraphyses strongly conglutinated, 2-2.5 µm thick, simple to sometimes forked, the apical cells slightly swollen, 3-5 µm wide; hypothecium colourless. Asci 8-16-spored, inflated-clavate, with a K/I+ blue tholus and a central non-amyloid area, Bacidia-type. Ascospores: (1-)3-septate, hyaline, straight or sometimes slightly curved, oblong-ellipsoid to fusiform-ellipsoid with abruptly rounded ends, 12-18(-23) x 4-6(-7) µm. Pycnidia rare, immersed, subglobose, the wall red-brown to dark-brown around the ostiole, colourless below. Conidia filiform, curved, 15-20 x c. 0.8 µm. Photobiont chlorococcoid. Spot tests: K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: a mild-temperate species found on the base-rich bark of isolated deciduous trees, especially Populus, Juglans and Ulmus; more frequent in the past, presently most common in southern Italy.