The following reference is currently the standard that is used for this Project: the taxonomy as it is laid out in the article Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005).
This taxonomy, based partly on cladistics, is actively under revision, and will continue to be for some years. The older taxonomies (for example, that based on the work of Johannes Thiele) are outdated and are no longer suitable to use. Please do not use terminology from any previous systems of taxonomy, and please update older taxonomy as you encounter it. We are part of the great "Tree of Life" Project, so the taxonomy is very important.