Ochrolechia parella (L.) A. Massal.
Pale grey-white thallus with white prothallus which can form large patches with zoned 'growth-rings' near thickened margin. Thick-rimmed, pinkish apothecia with the disk surface often covered by a thick, grey-white pruina. Widespread and often common on base-poor rocks and walls
Smith, C.W. et al. editors (2009) The Lichens of Great Britain and Irelandd
Dobson, F.S. (2018) Lichens: an Illustrated Guide to the British and Irish Species