Family: Rocellaceae
Opegrapha atra Pers.
Type: Crustose
Widespread but easily confused with other graphid lichens. Thallus creamy but can look quite green with algae present. Crowded lirellate apothecia
References:
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