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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