Collemopsidium foveolatum

Collemopsidium foveolatum is a species of lichen in the family Xanthopyreniaceae. It is a marine lichen that grows entirely embedded within calcareous substrates in the intertidal zone, particularly on limestone rocks and marine shells. The species is characterised by its minute, black, sunken reproductive structures (perithecia) that measure 0.10–0.24 mm in diameter and create small pits in the substrate. First described in 1911 from specimens collected in England, it has since been documented along coastlines in western Europe, North America and North Africa. The species can be distinguished from similar marine lichens by its consistently small, deeply embedded perithecia and its ability to maintain stable growth by keeping pace with natural substrate erosion....read more on Wikipedia.