Malmidea perplexa is a corticolous (bark-dwelling), crustose lichen in the family Malmideaceae. It was described in 2011 from northern Thailand. The species has a smooth, grey-green thallus without warts and ascospores that are non-septate and . It resembles Malmidea leptoloma but differs in having lighter-coloured apothecial discs and margins and a smooth thallus.
The species was introduced as Malmidea perplexa by Klaus Kalb in 2011 within a study on Malmidea and the family Malmideaceae. The holotype was collected on the descent from Doi Mon Larn to Mae Kampong village, east-south-east of Chiang Mai, in evergreen montane forest dominated by Lithocarpus, Quercus and Castanopsis; the specific epithet reflects that the senior author had known the species from Brazil for decades before finding it in Thailand again.
The thallus is crust-like, continuous and smooth, about 100âÂÂ250 üm thick, dull to slightly shiny and grey-green; soredia and isidia are absent. The medulla is whitish and KâÂÂ. The is with cells 6âÂÂ8 üm in diameter. Apothecia are , rounded, 0.4âÂÂ0.7 mm across and 150âÂÂ180 üm high; the is plane to slightly convex and brown-grey to sooty, bordered by a thin margin of the piperis type that is about 20 üm thick, slightly prominent and whitish-grey to dark brownish-grey. The is hyaline and internally lacks the medullary layer (about 30âÂÂ70 üm wide) and lacks hydrophobic granules; the is about 20 üm high and light brown, the hymenium 75âÂÂ120 üm and hyaline, and the 30âÂÂ80 üm and hyaline, KâÂÂ; the is indistinct. Asci measure 50âÂÂ65 à10âÂÂ15 üm. Ascospores number 6âÂÂ8 per ascus, are colourless, ellipsoid, non-septate and , 9âÂÂ13 à5âÂÂ7 üm with an about 0.5-üm . No lichen substances were detected by thin-layer chromatography or high-performance liquid chromatography.
The species grows on tree bark in evergreen forests in Thailand (including sites in Chiang Mai and Khao Yai and the Kaeng Krachan area) and is also recorded from south-eastern Brazil (São Paulo), at roughly 520âÂÂ1000 m elevation.