Verrucaria viridula (Schrad.) Ach.
Thallus crustose, thin, superficial to more or less immersed, areolate and divided by cracks, brown, brown-greenish to brown-grey, often with metallic green sparkles, in general delimited with a dark brown prothallus. Perithecia to half to almost completely immersed in thallus, 0.15-0.5 mm wide, appearing black, as convex to conical-hemispherical projections. Ascospores ellipsoid to broadly ellipsoid, perispore sometimes apparent, 28-34 x 14-20 µm. Chemical spot tests negative. On calcareous rocks, including limestone, mortar, and in the Finistère on kersantite (intrusive magmatic calco-alkaline rock commonly used for religious statuary). N.B. The status of this species is questionable since several taxa may be included; considered here sensu lato. Not to be confused with Verrucaria nigrescens on calcareous substrata, much more darker brown and more areolate with hypothallus visible in-between, and on mortar with Verrucaria ochrostoma that hardly may be distinguishable, but more greyish. In coastal areas on siliceous rocks, refer to Verrucaria fusconigrescens.*