Comparative descriptions were made for 23 species belonging to 23 genera of 17 families of Archaeogastropoda. Using data from new observations and previous literature, cladistic analyses were carried out based on a priori homology assumptions.

The analysis resulted in cladograms (length=209, CI=0.660, RI=0.881, RCI =0.582) having the following topology: Outgroups+(Patellogastropoda+(Caenogastropoda+(Neritopsina+(Neomphalus + (Cocculina + (Seguenzia + (Lepetodrilus + (Zeugobranchia+Trochoidea)))))))). Their relationships are summarized as follows: (1) In agreement with recent theories, Patellogastropoda are regarded as the earliest gastropod offshoot, because of retained plesiomorphies shared with outgroups. Especially, their stereoglossate radula and two-layered jaw in common with Tryblidiida or Cephalopoda suggest a basal position within Gastropoda. (2) Non-Patellogastropoda are united as the monophyletic group, Orthogastropoda Ponder and Lindberg, 1996. They are further divided into Rhipidoglossa and Caenogastropoda. (3) A robust clade is formed by genera of Neritopsina diversified from marine to terrestrial environments. (4) Neomphalus represents an early offshoot within the rhipidoglossate clade. Similarities were found between Neomphalus and Vetigastropoda in buccal musculature. (5) The position of Cocculinidae is suggested to be closer to Vetigastropoda than to Neritopsina. Monophyly of Cocculiniforrnia was not tested. (6) Monophyly is supported for Seguenzioidea, Lepetodriloidea, Zeugobranchia, and Trochoidea, and they are defined as Vetigastropoda(7) Architaenioglossa cannot be included in "Archaeogastropoda" because of apomorphies shared with Neotaenioglossa, so that they are united as the caenogastropod clade. (8) "Archaeogastropoda" are considered here as a "pre-taenioglossate" grade, including patellogastropod and rhipidoglossate clades.

Transformations in character evolution were revealed by a posteriori character evaluation under parsimony criteria: (1) Symmetry of the pallial complex in Zeugobranchia must be interpreted as secondary due to reversal. This view is supported by the ontogeny of the complex. The gastropod common ancestor probably developed asymmetrical pallial and internal organs similar to those of Patellogastropoda. (2) An amphineurous-type of primitive buccal musculature is retained in archaeogastropods, but in contrast wellorganized compact type evolved in the common ancestor of Caenogastropoda including Architaenioglossa. Digestive tract and radular teeth also show patterns for simplification from "Archaeogastropoda" to Caenogastropoda. Dietary change and proboscis formation probably played an important role. in the evolution of the gastropod digestive system. (3) Heart and coelomic organs were independently reduced on the right side, which generated a mosaic pattern of character states among archaeogastropods. Therefore, unpaired "mesogastropod" characters no longer imply the non-archaeogastropod nature of the taxa having such states.

By comparison with the anatomy-based hypothesis of relationships, phylogenetic evaluation of characters of the shell's protoconch was made: (1) Morphology of the archaeogastropod protoconch was categorized into four basic morphotypes, namely, symmetrically uncoiled, paucispiral, multispiral, and globular (modified terrestrial). Patellogastropods have characteristic mode of septal formation and loss of the protoconch. The internal resorption of the columella in protoconch and teleoconch is also a specific feature of Neritopsina. (2) Torsion at the larval stage has no relationship with shell coiling at least in the case of the symmetrical protoconch of Patellogastropoda. (3) The above categorization of protoconch morphology is largely consistent with categorization by anatomical characters, and therefore morphotypes of the protoconch (especially mode of coiling) can be regarded as phylogenetically essential shell characters. (4) The gastropod protoconch is presumed to have undergone evolution through longitudinal elongation from a cap-like conchiferan type (Patellogastropoda) into coiling forms by asymmetrical mechanical deformation (non-neritopsine Rhipidoglossa), and acquisition of the multispiral form by accretionary growth (Neritopsina and Caenogastropoda).