One of the functions of the perio is protection - such as protection for
burrowing molluscs who are constantly grinding their shells against sand and
silt.  Also, in colder climes, and in fresh water there is less CaCo3 available,
so the need to protect the shell from dissolving is correspondingly greater: the
perio simply insulates the shell from the low-carbonate water.  As far as the
beauty that often hides under the perio, i sometimes wonder if God didn't put it
there just for us....?

Cheers,
Ross.
--
Ross Mayhew: Schooner Specimen Shells: Http://www.schnr-specimen-shells.com
"We Specialize in the Unusual"
Phone: (902) 876-2241; Fax: (902) - 479 - 1863
But try to find "something for Everyone"!!
Snail Mail: 349 Herring Cove Rd, P.O Box 20005, Halifax, N.S., Canada, B3R 2K9.