Parmeliopsis ambigua  

Thallus yellow-grey to greenish, to about 40 mm diam. Lobes flat, closely appressed, to 1 mm diam and frequently palmately divided. The lobe tips have discrete soralia and they and the centre of the thallus become covered with concolorous powdery soredia. Lower surface black in the centre with simple stout rhizines that adhere firmly to the substratum, becoming light brown towards the lobe tips and lacking rhizines.
Anamorph: no information available.
Teleomorph:Ascomata very rare in Britain, with light brown discs. Ascospores 7-11 x 2-3 µm.
Chemistry: cortex K+ very pale yellow, KC ± yellow. Medulla UV+ white.