Arthonia atlantica P. James

    Thallus crustose, isolated but often forming mosaics with other species, very thin and less than 0.5 mm thick and cracked-areolate, sometimes thinly arachnoid in places, densely covered with minute crystals, white, whitish to pale grey, prothallus arachnoid, blackish. Apothecia very small, 1 x 0,10-0,15 mm, blackish,  ± immersed and hardly visible at least initially, broadly ellipsoid, irregular and elongated or ± stellate, often joining to form an irregular reticulum sharing an Enterographa aspect. Ascospores, turning brown when mature, 3- or 4-septate, oblong-ovoid, the upper cell enlarged, 16-24 x 6-7 µm. Thallus C-, K+ yellow, KC- , Pd- or Pd+ pale yellow or red (2 varieties have been described). Photobiont : Trentepohlia. Coastal, on hard siliceous rocks or walls below dry overhangs together and easily confused with  Peterjamesia circumscripta of which thallus is thicker, K-, but KC+ pale yellow and Pd+ yellow, and finally ascospores 4-(7)-septate. N.B. In fact, in the field considering macroscopic and chemical spot tests results, all the intermediate specimen between these 2 species may be found. Additional analyses would be required in order to clarify this issue. The chemical tests and the resulting colours are sometimes difficult to analyse due to the orange photobiont and to the fact that this species (as most of its neighbours) seems to be a parasite.*