Placopyrenium canellum (Nyl.) Gueidan & Cl. Roux

      Thallus crustose strongly adherent to the substrate, without apparent prothallus, areolate, 1-2 cm in diameter, but may form colonies, greyish, grey-blackish, fading in the sun, areoles 0.5-1 mm, angular, separated by wide black slits, flat to slightly domed, traversed by sinuous blackish lines. Perithecia completely immersed in the thallus, 1-4 per areole, 0.1-0.3 mm in diameter, ostiole inconspicuous, blackish excipulum. Spores simple, colourless, elliptical, with a perispore, 19-26 x 6-10 µm for the canellum morphotype, 10-18 x 5-8 µm for the small-spored morphotype according to the literature; 19-22 x 8-10 µm according to our measurements. Photobiont: Chlorophyceae. No color chemical reactions. Slightly thermophilic, saxicolous and calcicole species growing on sunny limestone rocks, initially parasitic on Aspicillia calcarea then free.

Cf. Placopyrenium fuscellum of lighter colour, with areoles often subdivided, with smaller spores without perispores and parasitic at the beginning of Verrucaria nigrescens.