Just as in marine shells, there are two reasons why you might want to put a
land shell in bleach - to remove organic matter from within the shell, and/or
to remove the exterior periostracum.  Some land shells have a fairly heavy
periostracum which does rather obscure the color and pattern of the shell
(Strophocheilus, Achatina, Acavus, Ryssota, Orthalicus, Pleurodonte, etc.),
and these species do look more attractive with the periostracum removed (just
as a Conus does).  These shells can be safely bleached.  Usually a weaker
bleach solution will do the job - the periostracum generally isn't as thick
and tough as in marine species.  Other land shells have a thin, fairly
transparent periostracum, which allows the shell colors and patterns to show
through.  However, such a periostracum does give a yellowish cast to the
shell, and may mute the colors somewhat, so sometimes it may be desirable to
remove even the thin periostracum.  This can be done in about 10 minutes in
bleach solution.  HOWEVER, this is where collectors run into trouble bleaching
land shells.  Some species have their color and pattern in the periostracum,
rather than the shell.  Others have colors and markings in both, and the
appearance of the natural shell is actually an overlay of the periostracal
pattern over the shell pattern.  Bleaching shells of this kind can provide
some interesting (or disastrous, depending on your point of view) effects.  A
green Helicostyla florida can be bleached without harm; yet the beautiful
green Helicostyla portei, if exposed to bleach, will come out as white as an
Ovula ovum.  It's color is entirely in the periostracum.  Incidostoma
confusum, a large operculate land snail from South America, appears bicolored
in natural condition - the upper half of the shell light brown, the lower half
dark brown.  After bleaching, it is pure white, with a pink apex.  In
Chrysallis virgata, the spiral, or transverse markings are in the shell, but
the axial markings are a periostracal overlay.  Bleach it and you have spiral
pattern only.  Helicostyla grandis in life appears light brownish with bands
and irregular splotches of dark chocolate brown.  After bleaching, it is
uniformly dark brown, fading to white toward the apex.  The light brown color
seen in life is periostracum, and the dark bands and "markings" are actually
uncolored, transparent areas of periostracum, where the underlying dark shell
color shows through.  My first personal experience of this phenomenon was with
Placostylus hargravesi, a beautiful, elongate, olive-colored shell with
intricate darker markings.  After bleach - pure white!  Some multi-banded
snails, like Liguus, may have some color bands on the shell, others on the
periostracum.  So, while most land shells can be bleached safely, if you
aren't sure, try your least desirable specimen first, and avoid unhappy
surprises.
 
Paul M.