Arthoniomycetes are a class

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of ascomycete fungi. It includes the single order Arthoniales. Most of the taxa in this class are tropical and subtropical lichen

Lichen

Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

s.

Systematics

Phylogenetic analysis supports the monophyly

Monophyly

In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 of this class. Dothideomycetes

Dothideomycetes

Dothideomycetes is the largest and most diverse class of ascomycete fungi. It comprises 11 orders 90 families, 1300 genera and over 19,000 known species. Traditionally, most of its members were included in the loculoascomycetes, which is not part of the currently accepted classification...

 is a sister group.

Characteristics

Taxa have apothecia, cup- or saucer- shaped ascoma in which the hymenium

Hymenium

The hymenium is the tissue layer on the hymenophore of a fungal fruiting body where the cells develop into basidia or asci, which produce spores. In some species all of the cells of the hymenium develop into basidia or asci, while in others some cells develop into sterile cells called cystidia or...

is exposed at maturity. These apothecia are bitunicate - with clearly differentiated inner and out walls.