Thanks to Stephanie for all the information.  I would take exception on one
point however.  Land snails do not hold up well to some of the cleaning
methods used on marine shells.  They are much more likely to crack when
frozen than most marine shells, though as Stephanie pointed out, there are
many fragile marine species which would also be prone to cracking if frozen.
But the main consideration I wanted to mention is bleaching.  Soaking in a
bleach solution is the single most common means of cleaning marine shells.
This can also be used on some land shells, but you have to be careful.
Bleach removes periostracum, the fibrous outer covering many snails produce.
In most marine shells, the periostracum obscures the colors and pattern of
the shell, and most collectors want to remove it.  In some land snails, the
same situation exists.  But in many other land snails, the periostracum is
thin and translucent, and the color and markings are in the periostracum.  A
specimen of the large, beautiful emerald green Helicostyla portei, if placed
in bleach for half an hour, will come out as white as an Ovula ovum.
Placostylus hargreavesi has a beautiful, intricate pattern pre-bleaching;
pure white afterwards.  Incidostoma confusum, a big operculate land snail
from South America, is light brown above, dark brown below - until
bleaching.  Then it is pure white, with a pink apex.  Likewise for many
other land shells.  Helicostyla pan is multi-banded before bleaching.  After
bleaching it is three-toned, white, tan, and chocolate brown, but with no
banding at all.  Some Chysallis species have both axial and transverse
markings.  After bleaching, tranverse bands only.  The pretty axial markings
are in the periostracum.  So proceed with caution!
Paul M.