Paul et al.,

I'm not an expert on shell asymmetry, but recall that bivalves coil too. The
coiling is just more subtle than in snails (see Raup and Stanley,
"Principles of Paleontology"). A few bivalve genera coil "the wrong way"
with regard to other members of their families, but they are not simple
mirror images of other genera, so the cause is probably not the same as in
snails. The "wrongway bivalve" I am most familiar with, the Cretaceous
glycymeridid Protarca, looks like a "morphed" version of its relative
Trigonarca. Where Trigonarca has a strongly triangular outline, Protarca is
equant, looking like someone squeezed it front to back.

Andrew K. Rindsberg
Geological Survey of Alabama
"If you eat a live toad for breakfast, nothing worse will happen to you all
day."


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