Cetrelia olivetorum  (Nyl.) W. L. Culb. & C. F. Culb. s. l.

    Thallus foliose, to 10-12 cm max. length, made of broad lobes 0.5-2 cm wide, rounded at apices, older parts of lobes with ± continuous thin border of discreet marginal soralia, pale grey-greenish ; upper surface smooth to slightly wrinkled, glaucous-grey, ± dark according to hydration, sometimes tinged brown, dotted with minute, white, punctiform or coma-shaped pseudocyphellae; lower surface black at center with a few black rhizines, smooth, simple, with a wide, often paler, rhizine-free zone towards the margin. Apothecia very rare. Photobiont: Trebouxia. Several taxa, considered as species, varieties or chemotypes by different authors, exist. All have a cortex C-, K+ yellow, Pd-, but they differ by medulla:

1) C-, K-, KC-, Pd-, I-, UV+ white-blue = Cetrelia cetrarioides (Delise ex Duby) W. L. Culb.& C. F. Culb. 

2) C-, K-, KC+ red, Pd-, I-, UV+ white = Cetrelia chicitae  W. L. Culb. & C. F. Culb.

3) C-, K-, KC-, Pd-, I-, UV- = Cetrelia monachorum (Zalhbr.) W. L. Culb. & C. F. Culb. 

4) C+ red, K-, KC+ red, Pd-, I-, UV-  = Cetrelia olivetorum ss. stricto.

    On deciduous trees in oceanic and humid woodlands, seldom on mossy rocks. Do not confuse with Plastimatia glauca devoid of pseudocyphellae and with medulla I+ blue, Parmotrema perlatum with mdulla K+ yellow, without pseudocyphellae and with marginal soralia bigger and more abundant and with Punctelia subrudecta displaying much bigger pseudocyphellae.*