> And, when did “concentric” become “commarginal?”

Technically, concentric ought to be a bull's eye, and thus strictly
speaking applies to certain opercula and some limpet-shaped shells,
but for almost all bivalves and many other taxa, the starting point of
growth remains at one edge and ornament parallel to the growth lines
is thus commarginal, not truly concentric.  Both the gastropod and the
bivalve Treatise on Invertebrate Paleontology have useful glossaries,
though not every idea therein on the best term has been generally
accepted.  Thus, it depends on whether you want to hit a bull's eye of
precision or are only marginally pedantic.

--
Dr. David Campbell
4830 University Blvd E H4
Tuscaloosa AL 35404
"I think of my happy condition, surrounded by acres of clams"

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