Opegrapha areniseda Nyl.

    Thallus crustose, thin to thick, sometimes poorly delimited or even divided, chalky at first, then powdery with white minute crystals, finally smooth, pale grey-whitish, pale greyish, pale grey-yellowish, tinged lilac-pink or purple (when fresh), prothallus unconspicuous, arachnidan, brownish. Apothecia rare, lirelliform, more or less abundant, sometimes clustered in specific areas, hardly sessile, elongate, seldom forked, 0.5-1.5 x 0.10-0.3 mm, black. Ascospores fusiform, 3-5-septate, 20-30 x 5-6 µm. Pycnidia numerous (especially when apothecia are scarce), visible as black dots, 0.1-0.4 mm. Chemical spot tests negative. On slightly acid or neutral soft rocks near the seashore (schists) and mainly on old walls, notably of chapels when the stones are altered or friable, aslo on mortar. N.B. This species is highly variable and old altered specimen turning dark brown or blackish are difficult to identify.