The origin and evolution of the gastropod order Neogastropoda was investigated using an iterative, two gene (18S rDNA and cytochrome c oxidase I) approach to phylogeny reconstruction. Partial sequences spanning approximately 450 base pairs near the 5' end of the 18S rDNA gene confirmed the monophyly of Apogastropoda and its two subclades, the Caenogastropoda (including Neogastropoda and Architaenioglossa) and the Heterobranchia, but were incapable of resolving relationships among neogastropod

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... milies, or between Neogastropoda and higher Caenogastropoda. The monophyly of Heterobranchia is additionally supported by the presence within this group of a large insert of variable length in the 18S rDNA gene in the region corresponding to the E-10-1 helix of the RNA molecule. Cytochrome c oxidase I sequences were able to resolve fully the relationships among representatives of ten families of Neogastropoda. Maximum parsimony, maximum likelihood and neighbor-joining analyses of these data all revealed that Buccinoidea and Muricoidea [sensu Thiele, 1929] each represent a clade, while the families assigned by Thiele and some subsequent authors to the superfamily Volutoidea comprise a grade. Although the two toxoglossan taxa included in our study emerged as a grade rather than a clade, denser taxonomic sampling of this group will be undertaken to investigate further the paraphyly of Conoidea. Based on percent transversions at third codon positions of the CO I gene, differences among neogastropod families as well as those between the neogastropod families and Cerithium are comparable to genetic differences between orders of mammals, but are only slightly greater than differences between genera of penaeid shrimp.