Nora,
 
Thanks for the kind words, pretty exalted company!  As for being a
beginner, we are most of us really still beginners.  The subject matter
is vast, changes regularly, and is constantly growing as folks learn
more from deeper and deeper in the oceans.  One of the amazing things
about Conch-L is we have ready access to many experts.  It is like being
back in school with no worries about grades or papers.
 
As for the periostracum and operculum -- both are fascinating parts of a
shell.  I won't repeat what was said about the periostracum but would
like to add that some shells are better kept with the periostracum for
color, pattern, or interest.  Since I know you have access to Eisenberg,
check out the Conus doreensi on page 137 (figures 1 & 1a, plate 119).
This shows the shell with and without the periostracum.  Because it has
a more interesting pattern with the periostracum, this is one cone
usually offered by dealers without being soaked and cleaned in Clorox.
Another great periostracum is the Cymateum parthenopeum on page 83
(figures 13 & 13a, plate 65).  It is not a great picture but you can see
the hairy periostracum on fig. 13a and the clean and colorful shell on
fig. 13.  One last example, Trichotropis cancellata on page 56 (fig. 20
& 20a, plate 38).  In this case both figures show the periostracum
because that is what the shell is renown for, thus the common name
cancellate hairy shell.
 
On to the operculum.  This little "trap door" may have begun as some
sort of protection, either against predators or against desiccation.
But as was previously mentioned, some shells do not have an operculum
while others have only a small remnant operculum.  So at one extreme we
have the Neritidae and the Turbinidae with a hard, calcareous operculum
that closes the aperture tightly shut; then a vast number of shells with
a lighter, chitinous operculum that still completely closes the aperture
like the Trochidae and Naticidae; then shells with only a small,
probably nonfunctional operculum like the Conus (for simplicity, I'm
obviously ignoring many different examples like the Strombidae with a
serrated, claw like operculum used for locomotion); and finally shells
with no operculum like the Cypraea.  So like the previous e-mail that
dicussed the many uses of the periostracum; the operculum is also much
more complex than it first appears.
 
These two organs or parts of the shell are intriguing and worthy of
study all on their own.  For me this is part of the fascination of
mollusks.  Add to this the specialized radula (spear gun, scraping tool,
drill, grinder), the various ways of extracting oxygen, the different
shell designs, the beautiful nudibranchs without shells, the
intelligence of the Cephalopods, the ability to inhabit almost any
environment, the many strategies for reproduction, etc.  They are
incredible.
 
Tom Eichhorst in New Mexico, USA (looking in wonder at two very similar
looking xenophora species with two very different operculums)