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

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