Prosobranchia

thumb|Shells from a variety of prosobranch gastropods, from Ernst Haeckel's [[Artforms of Nature, 1904.]]

Prosobranchia was a large taxonomic subclass of gastropods, which included sea snails, land snails and freshwater snails. This taxon dates back to the 1920s, but has since been proven to be polyphyletic (consisting of more than one lineage of descent). In biology taxonomy must reflect phylogeny and monophyletic groups are preferred; in other words, the classification of a group must reflect its evolutionary descent as far as that is known through studies of Morphology (biology) and genetic analysis, and must include a common ancestor and all of its descendants. Therefore, the taxon Prosobranchia with its multiple evolutionary origins is no longer considered suitable to be used.