Thallus immersed, visible only immediately adjacent to the apothecia, very thin, c. 0.05 mm thick, pale brown, flake-like. Photobiont not observed.
Apothecia black, lecideine, orbicular, sessile, mostly constricted below, 0.4-0.7mm diam., disc flat with persistent raised margin, 0.05-0.1 mm wide Excipulum of radiating hyphae with enlarged cells (8.5-15 Ám diam.), with red-brown (K+ purple) pigment on outer walls of the hyphae, continuous under hypothecium. Hypothecium 35 Ám, dark blue, composed of randomly arranged hyphae. Hymenium hyaline but with vertical, reddish-brown (K+ purple) streaks in thick section, 100-120 Ám; epihymenial zone dark blue-green (K+ blue, N+ red), 30-35 Ám thick; subhymenium hyaline,15-20 Ám. Paraphyses simple, readily separating, 1.5-2.0 Ám thick, apices swollen to 4.0 Ám with dark-blue pigmented cap. Asci clavate, 85-90 x 35 Ám, Lecanora-type. Ascospores hyaline, 1-septate, (25-)28-36 x (11.5-)14-17 Ám.
Conidiomata not observed.
Chemistry: no substances detected by TLC.
Distribution: Known only from the type locality on Campbell island where it occurs on sandstone.
Notes: Megalaria macrospora differs from other species of the genus by its larger ascospores. The only associated species is a small, poorly-developed Placopsis sp.
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