Sea Ivory

Ramalina siliquosa, also known as sea ivory, is a tufted and branched lichen which is widely found on siliceous rocks and stone walls on coastlands round the British Isles, occasionally slightly inland. It grows well above the high-tide mark but is still very tolerant of salt spray. The branches are flattened and grey, and bear disc-like spore-producing bodies. It forms part of the diet of sheep in Shetland and on the coast of North Wales. It is found in Iceland where it has a conservation status of a vulnerable species.

Excerpt from the Wikipedia article Ramalina siliquosa, by its authors, under the CC BY-SA 4.0 licence.