Harry, thanks very much for the excellent explanation of an amazing natural
process.

The final paragraph of your reply refers to the "final varix." Are you
saying that the number of varices is predetermined (determinate)? That at
some point the animal doesn't try to grow its shell any further because
somehow it knows that it cannot because doing so would result in some
insupportable imbalance between factors such as the volumes of the cavity
and living tissue, locomotive power vs the mass of the shell, shell
accretion potential, etc.?

I intuit that snails with indeterminate growth hit some sort of mathematical
limit in size, but that the demands of growing varicate shells are extreme
and that there is some quantum physics of sorts at work.

By the way, now that I have the vocabulary, I found another post on CONCH-L
about intervarical specimens
http://www.listserv.uga.edu/cgi-bin/wa?A2=ind0312d&L=conch-l&D=0&P=6166 and
attributes their rarity (at least in some species) to collector bias.

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