Description: Thallus crustose, episubstratic, thin, whitish to yellowish white. Apothecia arthonioid, lirelliform, brown-black, semi-immersed in the thallus, 0.5-2 mm long, branched to stellate, with a mostly flat, grey-pruinose or rarely epruinose disc. Epithecium blue-black, rarely dark olive-brown, K+ greenish; hymenium colourless to pale brown, 50-75 µm high, I+ blue; hypothecium colourless to pale yellowish. Asci 8-spored, broadly clavate, semi-fissitunicate, with a large apical dome and a distinct ocular chamber, Arthonia-type. Ascospores (2-)2-3(-4) septate, hyaline, more or less fusiform, 9-18 x 2-3 µm, the apical cell not enlarged. Pycnidia common, black but often white-pruinose, slightly emerging from thallus. Conidia bacilliform, 5-6 µm long. Photobiont trentepohlioid. Spot tests: thallus K- or K+ yellowish, C-, KC-, P-. Chemistry: unidentified substances in thallus.
Note: a mild-temperate, suboceanic lichen found on old oaks, in dry parts of the trunks seldom wetted by rain, in parklands and open woodlands. The record from Veneto (see Nimis 1993: 80) requires confirmation. The species is included in the Italian red list of epiphytic lichens as “Critically Endangered” (Nascimbene & al. 2013c).
Growth form: Crustose
Reproductive strategy: mainly sexual
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)