Zwackhia viridis (Ach.) Poetsch & Schied.

    Thallus crustose, very thin to insconpicuous, 2-3 cm across, potentially forming large colonies, smooth, pale greyish, greyish, pale brownish, often appearing bright green-yellow owing to microalgae populations, notably towards apothecia. Apothecia lirellate, numerous, partially immersed in the thallus, little protruding, rather short, simple, seldom branched or ellipsoid, 0.5-1.5 x 0.2-0.4 mm,blackish, disc hardly visible at the bottom of fissures. Ascospores acicular, 8-15 septate, 23-60 x 5-9 µm. Thallus: C-, K- (sometimes brown-reddish due to bark reaction), KC-, Pd-. Rather rare, on bark of deciduous trees, notably those with smooth bark in humid oceanic woodlands. N.B. Zwackhia (Opegrapha) sorediifera is very similar but with orange-ochre soralia, C+ pink-orange, KC+ red. See also Zwackhia prosodea that is similar with short and protruding lirellae, grows on the dry bark side of old oaks and yews.  Not to be confused with Opegrapha viridipruinosa of which lirellae are widely opened and the yellow lemon powdery disc conspicuous.