Diplotomma alboatrum  

Thallus thin to thick, often wide-spreading, white or pale (rarely dark) grey, smooth or rimose-cracked or granular, ± determinate. Cortex densely packed with minute crystals, not dissolving in K. Dark prothallus absent. Photobiont chlorococcoid.
Anamorph: not known.
Teleomorph: ascomata apothecia, 0.3-0.8(-1.5) mm diam., emergent; disc at first flat, later ± convex, often strongly pruinose. Thalline exciple sometimes present but usually becoming excluded, white, sometimes ± crenulate. True exciple inconspicuous, continuous with the hypothecium. Epithecium brown. Hymenium 45-75 μm, tall, hyaline, I+ blue. Hypothecium brown, almost opaque. Interascal tissue of simple paraphyses with enlarged, pigmented apical cells. Asci (40-) 50-65 (-70) × (10.5-)13-20 (-25) μm, clavate to clavate-ellipsoidal, Lecanora-type, 8-spored. Ascospores arranged bieseriately, (11-) 15-20 (-30) × (5.5-) 8-10 (-17) μm, at first (1-)3-septate, mostly submuriform at maturity, dark greenish brown, smooth or faintly verrucose, without an epispore, gelatinous sheath or appendages.
Chemistry: Thallus C–, K–, KC–, Pd–, UV– (no lichen substances detected by TLC).