The lichen genus Diplotomma introduced in 1849 by Flotow has frequently been subject to taxonomic debate. Originally proposed as a segregate of Lecidea because of a double exciple, it soon became more widely accepted when Massalongo emphasized the pluriseptate ascospores as the main character of the genus. Modern checklists and floras are divided on whether to treat the group as part of Buellia which has been characterized in the strict sense by one-septate ascospores. Nordin criticized the classical concept and recently confined Diplotomma to a group of species with calcium oxalates in the thallus, moderately pruinose apothecia and a conspicuously thickened perispore. Material of Buellia subalbula from the Sonoran Desert Region was examined with the light microscope, and the scanning and transmission electron microscope. Calcium oxalate content of the thallus was studied with x-ray diffractometry and specimens were analyzed using thin layer chromatography. The species has not previously been reported from North America and was commonly misidentified as B. retrovertens Tuck., a taxon recently synonymized with B. dispersa A. Massal. Buellia amabilis, a taxon described by de Lesdain from Mexico is most likely a later synonym of B. subalbula. The species is characterized by a thickly crystalline-pruinose thallus with abundance of calcium oxalates, moderately pruinose apothecia and a thickened perispore. These characters are consistent with Nordin's concept of Diplotomma s. str. even though the perispore is only moderately thickened. However, B. subalbula has one-septate ascospores and only a proper, not a thalline exciple.

Key words: Buellia amabilis, Buellia subalbula, Diplotomma, North America, Sonoran Desert Region, Taxonomy