Cliostomum griffithii (Sm.) Coppins

    Thallus crustose, more or less thick, smooth, continuous or rimose, sometimes cracked, whitish, pale grey-whitish, pale grey-yellow. Apothecia usually in small numbers, scattered or in small clusters, 0.2-1.5 mm diam., disc flat then convex, colour variable according to age, pinkish, grey-violine, pale brown-purple, brown, dark brown slightly black, finally often blackish more or less whitish powdered when fresh, margin paler persistent finally excluded. Ascospores colourless, broadly ellipsoid and a little lozenge-shaped, (0)1(3)-septate, with sometimes oily microdroplets in each cell, 8-16 x 3-5 µm according to the litterature, 8-13 (15) x 3-4 µm according to our measurements. Pycnidia usually highly numerous (inversely proportional to apothecia), large, immersed or sessile, but most often in a swelling of the thallus, black, simple opening: circular to oblong, sometimes multiple; these numerous openings may suggest an Aspicilia or a Diplotomma, the wall of these pycnidia is brown-purple in KOH. Photobiont : Trebouxia. Thallus : K+ pale yellow, KC-, C-, Pd- or +/- pale yellow ; pycnidia :  K+ magenta or purple-red (be careful, the pycnidia are so numerous that they might give the impression that the thallus is turning K+ purple!). Rare in our prospection area, on (acid?) bark of the dry side of trunks, notably coniferous, sometimes directly on the wood of the peeled trunks and even the worked wood, see: Cliostomum griffthii forme lignicole.

N.B. Cliostomum corrugatum has nordic maritime trends, a thicker thallus, less dark pycnidia and grows on worked woods of maritime buildings, coastal jetties or near the coasts.