Diplotomma glaucoatrum (Nyl.) Clauzade & Cl. Roux

    Thallus crustose, rather thick, more or less verrucose to areolate, pale grey-glaucous, pale grey-ochre, clayey, sometimes with sections pale brown or dull whitish or rusty on iron-rich substrata, effuse with indistinct prothallus except on flat surfaces where a thin grey line may be observed. Apothecia numerous, 0.3-0.8 mm diam., pruine absent or very slightly pruinose, immersed at first then slightly prominent, thalline exciple absent or inconspicuous, brown, disc black, flat at first then convex, exciple distinct vanishing when old. Hypothecium pale brown. Ascii 8-spored, 60-65 x 18-22 µm according to our measures. Ascospores dark brown-black, 3-septate at first then submuriform, 12-22 x 7-11 µm  (14-18 x 7-9 according to our measures). Thallus: C-, K-, KC-, Pd-. Coastal species, apparently restricted to the Western coasts of France, on calcareous or weakly acid rocks near the seashore. N.B. The Clauzade's inerpretation is retained here. This taxon is sometimes ranked as a form of the very close Diplotomma alboatrum.