Candelariella medians

Candelariella medians is a species of crustose lichen in the family Candelariaceae. It was first described in 1862 by the Finnish lichenologist William Nylander from specimens found growing on stones in Paris's Jardin du Luxembourg. The species forms distinctive yellow, radiating crusts up to 3 cm across on lime-rich surfaces affected by human activity, particularly in churchyards and on old buildings. It is abundant in southern and eastern England but occurs throughout Europe, typically growing on calcareous masonry such as limestone memorials, church walls, and gravestones....read more on Wikipedia.