Parmeliopsis ambigua (Wulfen) Nyl.
Foraminella ambigua (Wulfen) Fricke Meyer
Thallus foliose, rosette-forming, to 3-5 cm diam., tightly appressed, lobes to 0.5-1 mm wide, usually with tips indented and divided, upper surface smooth, pale grey-yellowish when dry, pale yellow-greenish when moist, external lobe margin pale brown, numerous small soralia towards the lobe margin, isolate, contrasting with large subglobose soredia towards the thallus center, lower surface brown-blackish with rhizines, simple, concolourous, paler at tips. Apothecia very rare, 1-2 mm diam., disc flat, light-brown, exciple concolourous to the thallus, wavy. Ascospores simple, slightly kidney-shaped, colourless, 7-12 x 3-4 µm. Photobiont: Chlorococcoid. Cortex: C-, K+/- faint yellow, KC+/- yellow, Pd-; medulla : C-, K-, KC-, Pd-, UV+ white. On acid bark of trees (coniferous notably), seldom on wood (and in that case together with Hypocenomyces scalaris) or on rocks. Imshaugia aleurites is rather similar and shares the same habitat, but is covered with isidia and Pd+ orange. Not to be confused with Hypogymnia farinacea of which the thallus and soralia are wrinkled, and medulla KC+ red. On rocks, not to be confused with Xanthoparmelia mougeotii that is Pd+ orange. N.B. Parmeliopsis hyperhopta, stricto sensu, is a very close species, but more grey and more often fertile, and its habitat is supposed to be montane or northward. It has been mentioned in Brittany (Crozon), and we hypothesize a possible confusion with Parmeliopsis ambigua.*