Picture taken on the 16th March 2008, near Plana Novella, Garraf Mountains, Olivella, Barcelona, on hard limestone, vertical position, well-lit place.
It is a very common lichen in Catalonia, always on hard limestone.
Crustose lichen, thallus almost white with a very thin black prothallus (see right corner). Apothecia immersed in the rock, regulary arranged but not in clusters, from red-brown to alost black mainly when dry, small, up to 0,5 mm diam., pruinose disc, grey pruina. It looks like a Verrucaria, but paying attention it is easy to see the lecideine apothecia.
See good drawings in Pascale Tiévant (2001) Guide des Lichens, Delachaux et Niestlé, pag. 118.