Wahlenbergiella (Verrucaria) mucosa

A very common species on the shore. The thin superficial thallus is a smooth olive-to-dark green (sometimes black), weakly translucent when fresh and wet, with a distinct white prothallus. The perithecia are immersed.

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