by Dr. Gary Rosenberg

Classification of the Mollusca

The following classification reflects many changes in ideas of molluscan relationships in the last fifteen years, particularly among gastropods. The subclass Prosobranchia, which is normally considered to include all the gastropods except the pulmonates and opisthobranchs, is not recognized. Biological groups should include the common ancestor and all of its descendants, but when defined in this way, Prosobranchia becomes a synonym of Gastropoda. Thus, an increasing number of biologists, prefer to use prosobranch as a descriptive term, rather than as the name of a taxon. The Archaeogastropoda suffer from the same problem. Some workers restrict the archaeogastropods to the Pleurotomarioidea, Fissurelloidea, and Trochoidea, excluding the Cocculiniformia, and Patellogastropoda, others prefer the name Vetigastropoda for this group, to avoid confusion between a narrowly and a broadly defined Archaeogastropoda.

As shown in an evolutionary tree, there are many different major lineages of gastropods. If this tree is correct, it would be appropriate to consider each of these lineages a subclass. In the classification herein, they are ranked as orders, because no name of subclass rank has been introduced for some of them. In order to maintain consistency of ranks within the tree, Pulmonata and Opisthobranchia are called orders, although they are usually considered subclasses. The problem of non-equivalence of ranks results from various groups of workers with disparate traditions working on different groups of gastropods. At present it is impossible to present a classification that simultaneously gives consistent ranks and reflects current usage; this classification represents a compromise between the two.

The above material has been adapted from Dr. Rosenberg's The Encyclopedia of Seashells, published by Robert Halt, Ltd., London, 1992.