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).
Assessed by Woods & Coppins (2012) as of Least Concern. The species is widespread and (at least in this circumscription) has a wide ecological range.
Diplotomma alboatrum is an extremely variable species in need of a comprehensive revision. D. murorum and D. vezdanum may be lichenicolous forms of this species; their characters fall within the range of D. alboatrum. D. pharcidium is very closely related if not synonymous. D. chlorophaeum is also considered synonymous with D. alboatrum by some but has a distinct ecology (on usually siliceous maritime rocks, often associated with mortar) and chemistry (it is K+ yellow-red and Pd+ yellow-orange.
Widely distributed, throughout the British Isles and Ireland. BLS map here.
On calcareous rocks and mortar, and rarely on nutrient-rich bark, especially that of Ulmus and Fraxinus.