The question of the exact anterior-posterior axis can get especially
messy in that the shell and body may give different indications.  The
mouth-anus line, the hinge line, and the adductors are three obvious
possible choices that can be at a significant angle to each other.
It's especially bad in monomyarian or significantly heteromyarian
taxa.  For example, Tridacna has largely lost its front end.

Most groups of bivalves that attach by one side are fairly consistent,
but it varies between groups as to which side they use, and some
families show variation between species or even within species.

Ligament is usually posterior of the beaks.

Nuculids and nuculanids are among the few bivalves that frequently
have the anterior bigger than the posterior; usually if they are
different in size the posterior is bigger.

Water exits posteriorly in almost everything; it usually comes in
posteriorly but lucines and some others are an exception.

The foot is anterior.  The byssus is associated with the foot and so
provides a marker in some species.  E.g., in scallops the ctenolium is
anterior.

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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