White to greyish, thick and crazed thallus, with a black prothallus. Common on siliceous rocks but a difficult genus to identify. (Good tip from Neil Sanderson: K/UV + bright mauve fluorescence)

References: 
Wirth, V. et al. (2013) Die Flechten Deutchlands, Band 2
Smith, C.W. et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson, F.S. (2018) Lichens: an Illustrated Guide to the British and Irish Species