Wikipedia
Mesogastropoda was for many years a traditional taxonomic group of snails, an order. The order was composed mostly of sea snails, but it also included some land snails and freshwater snails, all of which were prosobranch gastropod mollusks.
This order was introduced by J. Thiele in his work from 1921, it and was used for many decades subsequently.
Recent research in malacology however has made it clear that Mesogastropoda was not a monophyletic taxon, and because of that, the taxon is no longer included in modern classifications. Nonetheless most of the standard texts and field guides on mollusks date from the time period when this classification was still current, and therefore references to mesogastropods or Mesogastropoda are frequently encountered.
The lower taxa that were traditionally contained in Mesogastropoda are now mostly placed in the superorder Caenogastropoda. A more detailed account of the taxonomy is given in the articles on Gastropoda and Archaeogastropoda, and a recent taxonomy is laid out in Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)