Llimonaea

Llimonaea is a genus of lichen-forming fungi in the order Arthoniales. The genus has been placed into the family Opegraphaceae. These lichens form thin, firmly attached crusts that vary from chalky white and grey to olive-green or dark brown, often edged by a neat black line and characterized by rounded or scribble-like reproductive structures that sit flush with the surface. Established as a genus in 1991, Llimonaea species are distinguished by their partnership with orange-tinged green algae, their spindle-shaped spores with multiple cross-walls that gradually turn brown with age, and their black-rimmed crusts with robust carbonized walls around the fruiting bodies....read more on Wikipedia.