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Caenogastropoda is a highly speciose and ecologically diverse subclass of Gastropoda but its higher order classification remains unclear, especially within its largest constituent group, Hypsogastropoda. Two nominal taxa encompassing most of the great diversity of Hypsogastropoda are in current widespread use: one is Latrogastropoda, which has been repeatedly redefined resulting in changes to the second, Littorinimorpha, which is generally not supposed to be monophyletic. We examined the utility of these divisions by assembling single-gene data sets of nuclear 28S ribosomal RNA (28S rRNA) and mitochondrial 16S ribosomal RNA (16S rRNA) and cytochrome c oxidase subunit I from many genera. Capuloidea was consistently found with strong support within Latrogastropoda, so this taxon is redefined here to include that superfamily. The analyses also suggested the redefinition of some superfamilies within Littorinimorpha, particularly for the clade comprising Truncatelloidea, Vanikoroidea and Rissooidea, and the Littorinoidea. Littorinimorpha was monophyletic (albeit lacking strong support) in the combined analysis of 28S rRNA and 16S rRNA and was resolved as the sister group of Latrogastropoda which was also monophyletic, with bootstrap support of 66%. Littorinimorpha was not monophyletic in other analyses. In these, the sister group of Latrogastropoda comprised clades of multiple littorinimorph superfamilies but these relationships were also not strongly supported.
