Eduard Heiman wrote,
> Perhaps one possible way to move forward with species problems is to go
from simple to more complicated: first agree on species definition from a
conchological point of view and after that to think about more complicated
biological aspects of the problem. In such a way we can at least gather
facts and understand better what we are talking about. I do not think that
number of species and subspecies "depends on your personal approach to
taxonomy and the purposes and methodologies of distinguishing between one
species and another." We need definitions of taxa, which are more or less in
consensus and understandable to most of us.

If I understand you correctly, you want a species definition that treats
shell characters ("conchological point of view") as more important than soft
parts ("more complicated biological aspects").

Some kinds of soft parts are reflected in the shell, such as the muscles and
siphons. And in fossils, usually the shell is the only part that is
available for study. But for living mollusks, it's a different situation.
Since some species have no shell, and some others cannot be distinguished
without observation of parts other than the shell, it is not possible to
construct an accurate taxonomy of mollusks without reference to the soft
parts.

Indeed, in the freshwater unionids, shell characteristics can be so variable
as to make DNA analysis necessary to classify species meaningfully. Just ask
David Campbell!

Shell characters can still be used to make identification keys, of course,
and visual observation of shells is still the most cost-effective way to
identify MOST mollusks. And great progress is being made as observers pay
closer attention to the early stages of mollusks, which are often quite
different from the adult stages, and are often relatively conservative. But
shell characters will often be trumped by soft parts, and certainly by DNA,
where the taxonomy of a species is in question.

Cheers,
Andrew K. Rindsberg

Geological Survey of Alabama

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