Many years ago I was anxious to find out more about lichens and so I bought the definitive guide to British lichens written by the all time specialist in the subject, Frank Dobson (the book is called 'Lichens: An Illustrated Guide'). This is part of what it says about Parmelia sulcata:
"Thallus orbicular, grey to glaucous-white. consisting of overlapped ridged and sinuate lobes, normally covered with a faint white reticulum along which soralia develops, these may spread and cover the centre of the thallus. Under surface very dark brown with black rhizinae. Apotheccia with a dark brown disc. Distinguished from P saxatillis by the presence of soralia as opposed to isidia."I have never mastered lichen in twenty plus years of trying and above is the reason why!
Read more: Lichen: Parmelia sulcata | Nature Notes from Dorset