Verrucaria muralis Ach.
Thallus crustose, usually very thin and superficial, allowing to see the colour of rock below, more or less immersed in the substratum, slightly cracked and minutely pitted, white-grey, brown-white, occasionally with greenish tinge when fully moist. Perithecia usually very numerous, varying in size and maturity, more or less immersed and prominent, forming shallowly convex to conical-hemispherical projections 0.24-0.50 mm diam., black, ostiole clearly visible, after the fall of perithecia small shallow pits can be observed at the substratum surface. Ascospores simple, broadly ellipsoid, colourless, 20-24 x 10-15 µm. Chemical spot tests negative. On soft calcareous rocks and pebbles and, frequently in our prospection area, on stones of walls weakly acid (schists) and mortar. N.B. On the field, on walls this species is difficult to distinguish from Acrocordia salweyi.*