Boloria dia (Linnaeus, 1767)
Clossiana dia (Linnaeus, 1767)

The generic placement of this species is discussed controversially. The most recent treatment of the phylogeny of Boloria and related (sub)genera by Simonsen et al. (2010) recommends to recognize a large genus Boloria, and to recognize monophyletic subgroups of Boloria as subgenera. This concept is followed by almost all authors. I adopt this concept here as well, but I think the recognition of two separate genera Boloria and Clossiana is also a viable evolutionary hypothesis that should be seriously considered in future analyses of the phylogeny and taxonomy of this group of butterflies for the following reasons:
(1) The analysis by Simonsen et al. (2010) shows that Boloria sensu lato is subdivided into two main clades, that are separated by deep branches. Thus, there are clearly two main evolutionary lineages, and these correspond to Boloria (sensu stricto)+Proclossiana, and Clossiana.
(2) These two clades may be regarded as subgenera or genera and this decision is, of course, subjective. However, the deep split between these clades should be given some significance when assigning a rank to the clades and should also be mirrored in the classification. Thus, genus rank better reflects the deep split between the clades than subgenus rank.
(3) The same rationale has been followed by De Moya et al. (2017) in their analysis of the phylogeny of another fritillary butterfly group (Argynnis sensu lato). They find similar deep splits between main clades and recognise these main monophyletic groups as genera not subgenera.
(4) Finally, the age of the split between the genera Argynnis, Fabriciana and Speyeria as recognised by De Moya et al. (2017) is around 8-10 million years. The age of the split between the two major lineages in Boloria sensu lato is approximately 13 million years and thus significantly older.
These are arguments for recognizing the two main clades in Boloria sensu lato as two separate genera, Boloria and Clossiana. Another study by Yakovlev et al. (2018) contradicts this notion, because in this study the species that would be placed in Clossiana did not form a monophyletic clade. The study of Yakovlev et al. (2018), however, might suffer from artefacts within the phylogenetic analysis, because (a) it was based only on short sequence fragments of the COI gene, (b) did not include species from Boloria sensu stricto, and (c) chose only a remotely related outgroup (a species of Coenonympha) for rooting.