<p>This dissertation is primarily focused on the <i>Buellia</i> species with pluriseptate spores, often referred to the section or genus <i>Diplotomma</i>, and secondarily on the Physciaceae. An ultrastructural study at family level gives new information on spore characters. Variations in the composition and structure of the spore wall layers and septa are described.</p><p>A phylogenetic analysis of the Physciaceae, based on morphological and chemical characters, is presented. In the resulting trees the species with pluriseptate spores appear in a poorly resolved clade together with other crustose species with 1-septate spores. A subsequent analysis, excluding foliose and fruticose species, shows that the species with pluriseptate spores do not form a monophyletic group, and that only a few are grouped together with the type species of <i>Diplotomma</i>. Due to low resolution, poor branch support, and uncertainty concerning the typification of <i>Buellia</i> all species treated are retained in <i>Buellia</i>.</p><p>Altogether 35 species with pluriseptate spores are treated, including ten new to science, viz. <i>B. aeruginosa</i>, <i>B. morsina</i>, <i>B. muriformis</i>, <i>B. oidaliella</i>, <i>B. pallido-marginata</i>, <i>B. romoletia</i>, <i>B. rubroreagens</i>, <i>B. terricola</i>, <i>B. tombadorensis</i>, and <i>B. tri-septata</i>. Historical outlines are presented, and information on life strategy, morphology, chemistry, ecology, and distribution is given. All species are similar and macroscopically not easily distinguished, having crustose thalli and blackish apothecia, but there is a great variation in microscopic characters, secondary chemistry, substrate preferences, habitat ecology, and distribution range. Several names are typified and many synonyms are listed. References are given to all species referred to <i>Diplotomma</i> in literature.</p>

This dissertation is primarily focused on the Buellia species with pluriseptate spores, often referred to the section or genus Diplotomma, and secondarily on the Physciaceae. An ultrastructural study at family level gives new information on spore characters. Variations in the composition and structure of the spore wall layers and septa are described. A phylogenetic analysis of the Physciaceae, based on morphological and chemical characters, is presented. In the resulting trees the species with pluriseptate spores appear in a poorly resolved clade together with other crustose species with 1-septate spores. A subsequent analysis, excluding foliose and fruticose species, shows that the species with pluriseptate spores do not form a monophyletic group, and that only a few are grouped together with the type species of Diplotomma. Due to low resolution, poor branch support, and uncertainty concerning the typification of Buellia all species treated are retained in Buellia. Altogether 35 species with pluriseptate spores are treated, including ten new to science, viz. B. aeruginosa, B. morsina, B. muriformis, B. oidaliella, B. pallido-marginata, B. romoletia, B. rubroreagens, B. terricola, B. tombadorensis, and B. tri-septata. Historical outlines are presented, and information on life strategy, morphology, chemistry, ecology, and distribution is given. All species are similar and macroscopically not easily distinguished, having crustose thalli and blackish apothecia, but there is a great variation in microscopic characters, secondary chemistry, substrate preferences, habitat ecology, and distribution range. Several names are typified and many synonyms are listed. References are given to all species referred to Diplotomma in literature.