Thelotrema lepadinum (Ach.) Ach.
Variable, shiny greyish-cream crustose thallus. Numerous apothecia in barnacle-like raised warts with large central opening. On smooth and sometimes rougher bark of deciduous trees
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species