Parmelia saxatilis (L.) Ach.
Common on a variety of substrates from siliceous rocks to acid-barked trees biased towards the north-west. Typically pale grey foliose thallus with conspicuous, white pseudocyphellae on the lobe surfaces. Often densely isidiate on older parts of the thallus, arising initially from the pseudocyphellae and arranged in a net-like pattern. Apothecia with orange-brown discs and isidiate margins
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species
Thell & Moberg, editors (2011) Nordic Lichen Flora, Volume 4: Parmeliaceae