Fissurina flavomedullosa

The thallus is crustose and grows on bark. It is green-gray, continuous, and up to 10 cm across and 60–120 μm thick, with a smooth to uneven surface and a dense cortex. The photobiont is from the green algal genus Trentepohlia; the photobiont layer has scattered clusters of calcium oxalate crystals. The medulla is yellow, and the base includes a strongly carbonized (blackened) layer about 20–30 μm thick.^[2]

The fruiting bodies (lirellae) are wavy (flexuose) and irregularly branched, ranging from sunken in the thallus to partly protruding (immersed to erumpent), and slit-like (fissurine), with a complete covering of thallus tissue (thalline margin). They are 1–2 mm long and 0.07–0.1 mm wide, with the disc hidden from view; the lips (labia) are thin and not frosted (non-pruinose). The spore-bearing layer (hymenium) is 80–100 μm high. Each ascus contains eight oval (ellipsoid), 4-celled (3-septate) ascospores measuring 10–13 × 5–6 μm. The spores have a thick outer wall and thick cross-walls, and stain violet-blue with iodine (I+ violet-blue). Chemically, the inner tissue (medulla) contains a yellow anthraquinone pigment (reacting K+ orange-red).^[2]

The species is known from the Los Amigos Research and Training Center (CICRA) in Madre de Dios, Peru, at about 270 m (890 ft) elevation in tropical lowland rainforest. The type collection was made on tree bark in secondary forest in August 2008.^[2]