Opegrapha cesareensis Nyl.

    Thallus crustose, rather variable according to substrata, usually thin, sometimes inconspicuous and evanescent especially for isolated specimen growing in sheltered areas below rock overhangs, smooth or slightly cracked, whitish, greyish, grey-greenish, pale brownish, etc. tinged purple, grey-lilac or lilac; typically delimited by a thin black or grey prothallus, tinged lilac in case of mosaics. Lirellae stubby, to 2 mm long, markedly prominent, scattered or in small clusters (but not in piles or heaps), most often simple and not branched, straight or curved. Ascospores narrowly ellipsoid, 4-5-septate, 15-22 x 4-5 µm. Pycnidia frequent an sometimes abundant, pycnospores to 6-8 x 1-1,2 µm. C-, K-, KC-, Pd-. Photobiont: Trentepohlia. In fissures and cracks on siliceous not too acidic rocks, dry or humid, most often in shaded areas of the xeric supralittoral zone (the presence of marine Verrucaria nearby, a lilac colour of thalli delimited by a grey-blackish prothallus within large mosaics -rather frequently observed- are positive clues to search this species in the field).

N.B. Rather variable species. In case of isolated specimen without blackish prothallus and without lilac tinge, observation of ascospores in required to distinguish from species belonging to the Opegrapha calcarea grp. Indeed, species belonging to this group may grow in the xeric supralittoral zone whereas Opegrapha cesareensis is restricted to this zone.