Arthonia calcarea  

Thallus thin, finely rimose-cracked or smooth, sometimes totally immersed, white, grey, pale green-pink or rusty ochraceous, often with abundant lipid-rich cells which stain in Sudan Black, 5-10 µm diam. Prothallus inconspicuous, soredia and isidia absent. Photobiont Trentepohlia.
Anamorph: conidiomata pycnidia, to ca 300 µm diam., frequently absent. Conidia 4.5-7 × 0.5-1.5 µm, bacilliform, or rarely ellipsoid, straight, hyaline, aseptate, thin- and smooth-walled, without an epispore, gelatinous sheath or appendages.
Teleomorph: ascomata apothecia, lirellate, very variable in form, 600-1200 (-2000) × (100-) 150-300 µm, 40-120 µm tall, sessile or rarely semi-immersed, black, not pruinose, scattered or contiguous, simple, sometimes branched or stellate or in complex clusters. Disc slit-like, sometimes partially exposed with age. Exciple K+ olive-green; epithecium brown, K+ greenish; hymenium 80-100 µm tall. Interascal tissue of pseudoparaphyses, septate, branched, often richly anastomosed, the apices not or rarely slightly swollen. Asci clavate-cylindrical, thick-walled and fissitunicate with a well-developed apical cap and ocular chamber, the lower part of the cap staining weakly blue with iodine, 8-spored. Ascospores 16-20 (-24) × 4-5 (-6.5) µm, clavate, 3-septate, hyaline, dextrinoid when young, fairly thick-walled, smooth, without an epispore, gelatinous sheath or appendages.
Chemistry: thallus C–, K–, KC–, Pd–, UV– (no lichen products detected by TLC).