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This puzzling species has been known to the first author for at least 20 years. As the generic position has been unclear, it has until now remained undescribed. It is superficially similar to other byssoid genera in the Arthoniales like Crypthonia, Dichosporidium, and Herpothallon s.lat. Crypthonia and Herpothallon both have thalli reacting K/I+ patchily blue in 0.2% and 1% aqueous iodine solution (Frisch & Thor, 2010; Frisch et al., 2010) and Dichosporidium has thalli that at least sometimes are sparsely squamulose, a chemistry with depsidones of the beta-orcinol series and when pseudoisidia are present, these are more compact (Thor, 1990). Otherwise, Dichosporidium nigrocinctum (Ehrenb.: Fr.) G. Thor resembles the new species in thallus morphology and coloration. The genus Sagenidiopsis in South America until now only includes S. undulata (Fée) Egea, Tehler, Torrente & Sipman (Egea et al., 1995; Cáceres, 2007), a species previously included in Byssophoropsis (Tehler, 1993). S. undulata has apothecia, lack pseudoisidia and has a similar secondary chemistry including lecanoric acid and an unidentified pinkish spot (Tehler 1993). The presence of diploschistesic acid in S. isidiata is surprising as this substance previously has only been found in Diploschistes spp.