A term I have also seen used (more corrently) than concentric is gnomonic.
Comarginal addresses the position of the structure whereas gnomonic
addresses the geometric process.

G. Thomas Watters, PhD
Curator of Molluscs
Department of Evolution, Ecology and Organismal Biology
The Ohio State University
Museum of Biological Diversity
1315 Kinnear Road
Columbus, OH 43212  USA
v: 614-292-6170
f: 614-292-7774

"The world is my oyster except for months with an "R" in them" - Firesign
Theatre

"I have the world's largest collection of seashells.  I keep it on all the
beaches of the world...  Perhaps you've seen it." - Steven Wright

Visit the Division of Molluscs at:
www.biosci.ohio-state.edu/~molluscs/OSUM2


"Let me take you, baby, down to the riverbed
Gotta tell you something go right to your head" - Spirit

So join the Freshwater Mollusk Conservation Society at:

http://molluskconservation.org/



-----Original Message-----
From: Conchologists List [mailto:[log in to unmask]] On Behalf Of
David Campbell
Sent: Friday, July 30, 2010 9:38 AM
To: [log in to unmask]
Subject: Re: [CONCH-L] Help with terminology

> 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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