Parmelina pastillifera

Parmelina pastillifera is a species of foliose lichen in the family Parmeliaceae. The species forms greyish rosettes typically 4–8 cm across on nutrient-rich bark, particularly on ash and maple branches, as well as on rocks and tiles in exposed situations. It is readily identified by its characteristic button-like, blue-black outgrowths (isidia) that cover the lobes and often become dense in the centre of the thallus. The lichen occurs in Europe and Great Britain, where it is associated with well-lit habitats....read more on Wikipedia.