Placopyrenium trachyticum (Hazsl.) Breuss

    Thallus squamulose, squamules flat, 1-1.5 mm across, crowded and almost with contiguous free edges, squamules margins black producing an overall areolate aspect, very pale brown-grey, covered with a grey-whitish pruine, overall aspect grey-whitish, ash grey, prothallus black. Perithecia totally immersed within the thallus, hardly visible as dark dots, 1-5 (10) per squamule. Ascospores broadly ellipsoid, colourless, simple, 13-20 x 5-7.5 µm. Photobiont: Chlorophyceae. Chemical spot tests negative. Rare, on calcareous rocks or mortar of walls in nutritent-rich zones. N.B. This species would be parasitic, at least at the beginning of its development, of Verrucaria and notably of Verrucaria nigrescens. Very similar to Placopyrenium fuscellum (and sometimes ranked as p.p. by some authors), which differs by its absent or very reduced prothallus.