J. Phys. Hist. Nat., 87: 203, 1818. Basionym: Verrucaria galactites DC. in Lamarck & de Candolle - Fl. Franç., éd. 3, 2: 315, 1805.
Synonyms: Arthonia galactites var. depuncta Nyl.; Arthonia marginella Dufour; Arthonia punctiformis var. galactina Ach. non Arthonia cinereopruinosa var. galactina sensu A. Massal.; Coniangium galactites sensu Bagl.?
Distribution: N - VG (Sundin 1999), Frl (TSB 6585), Ven, TAA (Nascimbene & al. 2007b), Lomb, Piem, Emil (Fariselli & al. 2020), Lig (Watson 2014). C - Tosc, Laz, Abr, Sar. S - Camp, Pugl, Bas (Nimis & Tretiach 1999), Cal (Puntillo 1996), Si (Ottonello & al. 1994, Grillo & Carfì 1997, Grillo 1998, Grillo & Caniglia 2004).
Description: Thallus crustose, white, mostly endosubstratic, without a distinct prothallus, facultatively lichenized, forming orbicular patches to 2 cm in diam. Apothecia arthonioid, immarginate, black, epruinose, circular to slightly elongate, flat to weakly convex, 0.2-0.8(-1) x 0.2-0.7 mm. Epithecium dark brown, K+ violet; hymenium pale yellowish green, c. 20-35 µm high, K-; paraphysoids 1-2 µm thick, the apical cells to 3 µm wide; hypothecium brown or yellowish green, the yellow green-pigment K+ violet, not dissolving. Asci 8-spored, broadly ovoid, semi-fissitunicate, with a large apical dome and a distinct ocular chamber, Arthonia-type. Ascospores 1-septate, hyaline, ovoid, (10-)12-15 x (3-)4-5 µm. Pycnidia usually numerous, black around the ostiole, with brown, K+ olive walls. Conidia long-filiform, curved, 10-16 x c. 0.5 µm. Photobiont absent or trentepohlioid. Spot tests: thallus K- or K+ pale yellow, C-, KC-, P-. Chemistry: no lichen substances detected.
Note: a mild-temperate species with optimum on the smooth bark of Fraxinus ornus, but also on Populus and even Pistacia, probably overlooked or/and confused with other species, but certainly not common, and perhaps mostly coastal or Tyrrhenian Italy. Most records, especially those from northern Italy, need confirmation.
Growth form: Crustose
Reproductive strategy: mainly sexual
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)