Verrucaria viridula (Schrad.) Ach.

   Thallus crustose, relatively thin and quite superficial, cracked and rimose or fissured, sometimes very scaly in very dry weather, pale brown, greenish-brown, grayish-brown in dry weather, pale grayish, a little greenish in wet weather, greenish-gray in very wet weather, generally limited by a thin, dark brown hypothallus. Perithecia buried in the thallus in a small pit, 0.15-0.5 mm, apex convex, blackish, involucrellum developed only around the ostiole and sometimes partially projecting, forming a small "beak". Spores simple, hyaline, broadly elliptical with sometimes a perispore, 28-38 x 14-20 µm. No characteristic color reactions. A slightly nitrophilous species found on limestone rocks or the mortar of walls (especially chapels), but also on Kersantite (calc-alkaline intrusive igneous rock) in Finistère.

N.B. While "species" appears to be a collective term that would include many taxa, it is used here in the broadest sense.

_ On limestone, it can be differentiated from Verrucaria nigrescens, which is much more blackish-brown and appears more areolated because the hypothallus is visible between the areoles. On mortar, it is much more difficult to differentiate from Verrucaria ochrostoma, which has a generally grayish and brownish coloration towards the top of the perithecium.

_ On siliceous rocks along the coast, consider Verrucaria fusconigrescens.