Conchology
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.
