Placopyrenium fuscellum (Turner) Gueidan & Cl. Roux

    Thallus crustose, 1-1.5 cm diam, potentially forming colonies, up to 5-7 cm across, more or less encrusted in the substratum, composed of squamules more or less angular, elongate or lobate (at thallus edge), 0.5-1.5 mm long, giving an areolate aspect, areoles (often subdivided) distinct owing to their black edges emphasised by the black prothallus visible in the slits between adjacent areoles, overall colour greyish, grey brownish. Perithecia immersed, 1-5 per squamule, visible only by their apices, plane or slightly convex, 0.1-0.15 mm diam., blackish. Ascospores ellipsoid, colourless, simple, 13-18 x 5-7 µm. Chemical spot tests negative. Rare, parasite of Verrucaria at first and notably of Verrucaria nigrescens, grows in the same habitats than the host, i.e. clacareous or weakly acid rocks, also on mortar of old walls. N.B. See also Verrucula hladuniana that is parasite of Caloplaca marina and grows in a different habitat and also Placopyrenium trachyticum that would, according to some authors, be synonymous (at least p.p.).

Cf. Placopyrenium canellum more thermophilic, with larger spores with a perispore and a parasite of Aspicilia calcarea.