Malmidea atlanticoides is a crustose lichen with a continuous thallus measuring 75–100
μm in thickness. The thallus surface is (warty), featuring more or less spherical that are 0.1–0.25 mm in diameter. The colour of the verrucae ranges from dull ash-grey to greenish-grey and light olive. Both
soralia and
isidia are absent in this species. The
medulla of the verrucae and the thallus is orange-yellow, showing
spot test reactions of K+ (orange to reddish) and P+ (
vermilion). The is with cells measuring 6–8 μm in diameter. The
apothecia of
Malmidea atlanticoides are
sessile and rounded, with a diameter of 0.5–0.8 mm and a height of 0.3–0.4 mm. The apothecial are flat to slightly concave, and their colour varies from beige to light brownish. The is of the
granifera-type, initially and becoming with age. It is whitish to cream-coloured, bulging, and towers over the disc. The is hyaline, while the medullary excipulum is filled with orange-yellow hydrophobic granules that are nubilous and dissolve in KOH, producing a lemon-yellow efflux. The base of the apothecium reacts to K with an orange-red colour. The is approximately 25 μm high and light brown, while the centrally located is 80–100 μm high, narrowing towards the margin, and dark brown without reacting to K. The is indistinct and the
hymenium is hyaline, measuring 100–110 μm in height. The
asci of
Malmidea atlanticoides are 60–80 μm long and 15–20 μm wide. Each ascus contains four to eight that lack
septa. These ascospores are broadly
ellipsoid, with equally thickened and walls, measuring 12–15 by 8–9 μm with a of 1–1.5 μm. Chemically, the species is characterised by the presence of
atranorin as a major component, along with an unknown
anthraquinone substance, detectable using
thin-layer chromatography. Its lookalike,
Malmidea atlantica, lacks both atranorin and the unknown anthraquinone. ==References==