The classification of the Gastropoda in three subclasses Prosobranchia, Opisthobranchia and Pulmonata had been set as a standard by Thiele (1929-1931) throughout the XXth century and is still presented in major textbooks (e.g. Brusca & Brusca, 2003).
Nowadays the classification of Gastropoda undergoes considerable reorganization in the attempt to bring it as close as possible to a changing phylogenetic hypothesis of the class. There is compelling evidence that Prosobranchia as classically understood is a paraphyletic taxon, and as a consequence it is being progressively abandoned. Even if one would adopt the standards of Evolutionary Systematics and tolerate paraphyletic taxa under some conditions, it would be embarrassing to maintain Prosobranchia at the same taxonomic rank as Opisthobranchia and Pulmonata, which together form a clade (Heterobranchia) which is at large the sister-group of Caenogastropoda (i.e. part of Prosobranchia).
The option taken herein is to derive the classification scheme as much as possible from Bouchet & Rocroi (2005, and references therein), with Linnean ranks added. This will be held as “basis of record” for all gastropod taxa even if those were already listed in previous versions of the database.
The taxa contained in the former Prosobranchia are distributed in separate subclasses (Patellogastropoda, Vetigastropoda, Cocculiniformia, Neritimorpha and Caenogastropoda) which are all supposed to be monophyletic. This has the incovenience of bringing a small group like the Cocculiniformia at the same rank as the large clade Heterobranchia (including Opisthobranchia + Pulmonata), but on the other hand has the advantage of being cladistically correct and of keeping Caenogastropoda and Heterobranchia at equal rank. Cases departing from the scheme of Bouchet & Rocroi (2005) will be explained in notes on the appropriate taxa. The initial split (Eogastropoda vs. Orthogastropoda) as in Ponder & Lindberg, 1997 is not retained, following Zapata et al. (2015)