Verrucaria muralis

A widespread and common species in Britain found most frequently on limestone and artificial substrates such as mortar, brick and calcareous soil.  Occasionally, also recorded on siliceous rock, The pale thallus is immersed or thinly superficial, the cortex typically lacking any pigments. Black perithecia are shallowly convex to conical-hemispherical without a layer of thallus.

Listed by Alan Orange in Flora of Glamorgan (Wade et al, 1994).