Description: Thallus foliose, loosely attached, forming wavy, wide-spreading, usually orbicular patches 6-20(-40) cm across. Lobes broad and rotund, (0.5-)1-1.5(-2) cm wide, with raised margins, greenish grey (sometimes tinged brown), pseudocyphellate; pseudocyphellae on upper surface small to rather large, usually not raised, often lacking in the central parts of thallus, those on the lower surface frequently present, at least on some ascending, contorted lobe apices. Soralia primarily marginal, elongated, usually very smooth and strongly convex, with fine soredia, 25-35(-40) µm diam. Lower surface black, wrinked, with scattered, simple, black rhizines and a brown, rhizine-free zone along the margin. Upper and lower cortices prosoplectenchymatous, with a non-pored epicortex, of densely agglutinated hyphae with tiny lumina; medulla white, with isolichenan, I-. Apothecia very rare, lecanorine. Asci 8-spored, Lecanora-type. Ascospores 1-celled, hyaline, ellipsoid, 12-15 x 7-10 µm. Pycnidia: unknown. Photobiont chlorococcoid. Spot tests: cortex K+ (weakly) yellow, C-, KC-, P-; medulla and soralia K+ yellow, C- or C+ faintly pink/violet, KC+ pale pink of pale reddish brown, P-. Chemistry: cortex with atranorin and chloroatranorin (in lower amounts than in soralia); soralia (and medulla) with atranorin, perlatolic acid (major), imbricaric acid (traces), 4-O-methylolivetoric acid (UV+ blue-white, best seen on TLC plates under short-wave light), and anziaic acid (traces), sometimes with a fatty acid.
Note: a species with the perlatolic acid syndrome plus traces of imbricaric acid, found on the bark of broad-leaved trees and on epiphytic mosses, more rarely on silicicolous mosses in humid, old, mostly montane forests. It is included in the Italian red list of epiphytic lichens as “Data Deficient” (Nascimbene & al. 2013c). See also note on Cetrelia olivetorum.
Growth form: Foliose, broad lobed
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)