Grey foliose thallus with rounded, overlapping lobes with irregular, incised edges. Isidia fragile, marginal and laminal, cylindrical or lobulate, often becoming coralloid-branched, brown-tipped and frequently with accompanying black cilia. Lower surface black with a broad, brown erhizinate marginal zone. Rhizines dense, simple. Apothecia rare, comprising brown discs with isidiate margins. On siliceous rock outcrops, in cliff-top heaths near coasts, and mossy tree trunks.

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
Thell & Moberg, editors (2011) Nordic Lichen Flora, Volume 4: Parmeliaceae