Carole,

I've found "Marine Molluscan Genera of Western North America: An
Illustrated Key" by Keen & Coan (1974) to be a very helpful reference.
The bivalve section is equally useful for the east coast, and it's
available on Amazon for about five dollars.

Colin

On Tue, May 15, 2012 at 5:30 PM, David Campbell <[log in to unmask]> wrote:
> It gets particularly confusing in some species, primarily those that
> are monomyarian, because different definitions of the
> anterior-posterior axis may be at large angles to each other.  Thus,
> one measure of "anterior tip" may be "anterioventral corner" by
> another measure.  The Treatise on Invertebrate Paleontology has a
> discussion (vol. N1).
>
> In heterodont bivalves, the dentition will be different in the two
> valves; however, somewhat analagous to sinistral gastropods, it is
> possible to have individuals or species with partial or complete
> left-right reversal of dentition.
>
> --
> Dr. David Campbell
> Collections Assistant
> The Paleontological Research Institution
> 1259 Trumansburg Road
> Ithaca NY 14850
>
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