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