A thin, encrusting lichen, mid to dark green, found on rocks of the lower intertidal zone. Apothecia are small and black, irregular and occasionally ridged. Easily confused with the commoner Verrucaria mucosa

References: 
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