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This lichen is very easily dismissed as H. revoluta (described elsewhere), which you will see looks very like it. H. britannica is a foliose lichen of well-lit acid rocks and moorland shrubs, especially at sites close to the sea. The jigsaw piece-like, blue-grey coloured, often ascending lengthy lobes should help distinguish this species as will the presence of the indigo blue to grey-black, sub-terminal, granular soredia. North and western Ireland; absent from the south-east.
Original text submitted by Vince J. Giavarini