Diplotomma alboatrum (Hoffm.) Flot. forme saxicole calcicole

   Thallus crustose, rather thick, smooth or uneven, rimose-cracked or granulose, heterogeneous, potentially composed of scattered fragments, whitish to pale grey, dark prothallus absent though a thin grey line may be observed in case of mosaics. Apothecia leceidine, immersed at first, 0.3-1 mm diam., mature and immature in combination, disc at first flat, later convex, black, often covered with a whitish pruine; thalline exciple sometimes present, white, sometimes ± crenulate due to thallus rupture. Ascospores brown, ellipsoid, at first 3-septate turning submuriform to muriform when mature, 15-20 x 8-10 µm. Photobiont: chlorococcoid. Chemical spot tests negative. Not especially maritime, but quite frequent on calcareous walls, calcareous or weaky acid rocks. N.B. This species is extremely variable and many taxa have being described according various interpretations of different authors, some on tree barks (Diplotomma alboatrum forme des écorces des arbres), other saxicolous on calcareous or acid rocks (Diplotomma alboatrum forme des rochers acides).  See also Diplotomma chlorophaeum which grows only near the seashore and K+ yellow > red and with a grey prothallus in thin and eroded thallus sections. On hard limestone see Diplotomma hedinii which is very similar but with a more homogeneous thallus and different ascospores (not submuriform). On the foreshore see Diplotomma glaucoatrum, which is very similar but with a less whitish thallus.*