Arthonia medusula (Pers.) Nyl.
Nyl. Mém. Soc. Imp. Sci. Nat. Cherbourg 5: 132 (1858)
Basionym: Opegrapha medusula Pers. 1810
Crustose, epi substratic, thin, whitish to yellowish white
Lower Side :
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
Thallus K- or K+ yellowish, C-, KC-, P-. Unidentified substances in thallus