To all,

There is another periostracum aspect, not yet mentioned.  In the Hawaiian
nerite Clypeolum granosum (Sowerby, 1825) the periostracum can take away the
pattern or leave it, depending upon how it is removed.  C. granosum is a
25mm, flat nerite with a series of granulations or bumps covering the shell.
The periostracum is dark brown, almost black.  When displayed as is, it is a
nice enough looking shell.  However, if the shell is allowed to dry the
periostracum will begin to flake off (as a periostracum is likely to do) and
the shell underneath the periostracum is solid white.  Simple enough, but
the story continues.  If, instead of allowing the shell to sit high and dry
and gradually lose the periostracum to expose the white shell underneath,
you decide to bleach it and speed up the process, the end result will be
quite different.  If immersed in bleach (and it takes a while as this is a
very persistant periostracum) the shell will come out as a black shell
covered with small white triangular spots.  Anywhere the periostracum has
already dried and flaked off, the shell will be solid white.  Ain't this
stuff great!

Tom Eichhorst in New Mexico, USA

P.S.  How about the land shell Chrysallis virgata (Jay, 1839) with the white
lightening streaks on the brown bands.  If you wet the shell, the white
streaks disappear.  When dry, they return.  I have personally never put one
of these in bleach to remove the periostracum, but I'll bet this process
would also deleat the white, moisture controlled, streaks.