Ponder and Lindberg (1997) in the book The Southern Synthesis, showed the Orthogastropoda as one of two subclasses of the Gastropoda, the other subclass being a very much smaller group called the Eogastropoda, which contained only 5 families of true limpets.
This subclass, Orthogastropoda, which one perhaps could call the true snails, was defined most concisely as all gastropods which were not members of Patellogastropoda, the true limpets.
The Orthogastropoda appeared to form a clade which was supported by unambiguous synapomorphies. These synapomorphies (a series of characteristics that appear in its members but not in the other forms it diverged from) were the identifying characteristics of the clade.
Some of the characteristics were:
eyes with a vitreous body on eyestalks.
paired jaws, with their position free from the buccal mass
a single kidney on the right side of pericardium
a flexoglossate radula (with a flexible radular membrane). The radula is the snail's tongue, used as a rasping tool.
unpaired osphradium (olfactory organ).
lateral ciliated zones of osphradium
a single left hypobranchial gland (on organ at gill, which releases secretions, such as the reddish dye Tyrian purple).
an unpaired ctenidium (a comblike respiratory structure in certain mollusks)
