Cliostomum tenerum (Nyl.) Coppins & S. Ekman

    Thallus small, 1-3 cm max. length, thin, inconspicuous, irregular and rugose, areolate, flat or irregular areoles more or less scattered but possibly clustered, cream, cream-greenish, pale grey-greenish with white margin, white prothallus. Apothecia starting immersed then emergent 0.2-0.4 mm diam., dispersed to sometimes contiguous, disc becoming more or less convex, sligthly pruinose, pale brown-greenish, greenish, pale brown, pale pink-brown, thin thalline exciple irregularly smooth-crenulate margin, cream. Pycnidia sometimes abundant, half immersed in the thallus. Ascospores fusiform-ellipsoid, straight to seldom slightly curved, pointed apices, simple to rarely 1-septate, 7-12 x 2-3 µm. Photobiont : Trebouxia. Thallus C- , K +/- yellow, Pd+ red (slow reaction in 10 min). Confined to dry, sheltered crevices and under overhangs of coastal siliceous rocks in the xeric-supralittoral zone. Discreet and inconspicuous, difficult to find due to its thallus weakly coloured and to the small size of its apothecia. See Tylothalia biformigera  that diplays ascospores with rounded apices and a green thallus, and Lecania atrynoides with bigger apothecia, brown-red to brown-dark, chemical spot tests negative.*