Synonyms: Allarthonia patellulata (Nyl.) Zahlbr.; Arthonia betuleti Nyl.; Coniangium krempelhuberi A. Massal.; Coniangium krempelhuberi var. effusum A. Massal.?
Distribution: N - Ven (Lazzarin 2000b, Lomb, Piem (Caniglia & al. 1992). C - Tosc (Putortì & al. 1999c, Loppi & al. 2002). S - Camp (Aprile & al. 2003b), Cal (Puntillo 1996).
Description: Thallus crustose, very thinly episubstratic, smooth to finely rugose, white, grey-white or yellowish, without a distinct prothallus. Apothecia arthonioid, rounded, sessile, 0.2-0.7 mm across, with a black, flat to slightly convex, epruinose disc, without a proper margin. Epithecium grey-green to olivaceous brown, 10-15 µm high, K-; hymenium colourless or yellowish green, not inspersed with oil droplets, 35-60 µm high, I+ reddish, K/I+ blue; paraphysoids branched and anastomosing, slender, c. 1.5 µm thick, not capitate, vertically oriented, without a brown cap; hypothecium dark brown to black, 30-80 µm high. Asci 8-spored, broadly clavate to subglobose, semi-fissitunicate, with a large apical dome and a distinct ocular chamber, Arthonia-type. Ascospores 1-septate, constricted at septum, with unequal cells (one subglobose, the other narrower and pointed), hyaline, obovate, 9-15 x 3-5 µm. Pycnidia black, sessile, c. 0.05 mm wide. Conidia bacilliform, straight or slightly curved, 3-6 x c. 1 µm. Photobiont chlorococcoid. Spot tests: K-, C-, KC-, P-. Chemistry: without lichen substances.
Note: a cool-temperate to boreal-montane, probably circumpolar species found on smooth bark, mostly of Populus tremula. The records from Peninsular Italy require confirmation. The species is included in the Italian red list of epiphytic lichens as “Vulnerable” (Nascimbene & al. 2013c).