Rock nail Calicium corynellum
An inconspicuous lichen with a diffuse yellowish-green thallus (the main lichen body) and small pinhead apothecia (the part that produces spores).
It grows over another lichen, Lepraria incana, and may be parasitic upon it.
Until recently it was only known from a single site in Britain, at Bywell in Northumberland. Since 1999 another three populations have been located, in Northumberland and Midlothian, but the original now appears to have been lost.
It has very narrow habitat requirements, being found on vertical or steeply sloping sandstone surfaces that are dry and protected from the direct effects of rainfall, and where competition from other lichens, algae and cyanobacteria is limited, but in a humid atmosphere where both this species and its host can survive without desiccation. This is generally in partial shade under a rock overhang or under trees, and often both.
The British populations are all in the artificial habitat of churchyards. These are managed sites and open to the public, so the lichen is subject to various threats of disturbance and the use of chemicals. An immediate threat is work to stabilize or reset leaning headstones to make them safe, work that is likely to be extremely damaging to this species.