Verrucaria nigrescens

A crustose species with a dark brown thallus that is regularly cracked into smooth areoles. Perithesia cover the thallus.

Verrucaria nigrescens is common throughout coastal and lowland Scotland where it occurs on well-lit calcareous rocks, walls and mortar, including gravestones. BLS distribution map

Verrucaria nigrescens can be parasitised by Placopyrenium fuscellum.

Verrucaria nigrescens on a wall at Pitmedden (NTS) (VC93) (note the Placopyrenium fuscellum in the upper half of this thallus)
Placopyrenium fuscellum on a church wall near Foveran (VC92)