File:A lichen - Rimularia furvella - geograph.org.uk - 1561543.jpg

English: A lichen - Rimularia furvella. This lichen was growing on a schist boulder beside a footpath above the shore (the boulder is located about 70 metres NW of 1358962, and stands out from the rocks of the beach, almost all of which are composed of red sandstone).

Rimularia furvella is a lichen that grows parasitically upon other lichens on exposed rocks; distribution map: http://data.nbn.org.uk/gridMap/gridMap.jsp?allDs=1&srchSpKey=NBNSYS0000018667 (showing that it is not particularly a coastal species).

Only the thick brown crust belongs to the Rimularia furvella; the grey lobes at the edges belong to the species that it is growing on. The brown crust is divided by cracks into smaller areas (called areoles). Through a lens, these areas, in turn, are seen to consist of densely packed rod-shaped outgrowths called isidia; compare: https://www.geograph.org.uk/photo/1050177

[Within many lichen species, genetic variation gives rise to several distinct chemical races (or "chemotypes"), producing different chemical compounds. The chemotypes are often visually alike, requiring chemical tests – http://www.thebls.org.uk/content/chemical.html – to tell them apart. Rimularia furvella has a race that produces gyrophoric acid, and another that produces norstictic acid; the specimen in the photo is of the latter kind.]