Helmut,
  There are two approaches, permanent marking or semi-permanent.  If you go
the permanent way you can use an archival fine line marker such as a PIGMA
penpr a ZIG pen.  You can also use india ink with a dip pen.  Just
remember, some india inks are considered a better archival product than
others.
  For a semi-permanent approach, write the catalog number on a 'small'
piece of archival (acid free) paper.  Then attach it to the shell in some
inconspicuous place.  It should be attached with an archival adhesive and
one that can be removed at a later date.  The following are relatively safe
adhesives: polyvinyl butyral (Butvar 76 and Butvar 98), polyvinyl acetate
(Vinac), and acrylic copolymer (Lucite, Acryloid B72, and Paraloid).

Regards,
Charlie

--On Saturday, January 27, 2001, 2:34 AM -0500 helmut nisters
<[log in to unmask]> wrote:r

> 'Dear Conchlers,
>
> what I never done before, I have to do now. To spare some plastic boxes
> for question of cost I have to put together sometimes the same species of
> a shell, but
> with some other locality. So I have to sign the shell with a number to
> separate them.
> Which fineliner or what kind of pen (writing utensils) you would consider
> to take. I told
> you that I never have done this before, but now it is necessary to do.
> with best shelling greetings and thanks for a quick answer
> Helmut
>
> Helmut "Helix" Nisters
> private:
> Franz-Fischer-Str. 46
> A-6020 Innsbruck / Austria / Europe
> phone: 0043 / 512 / 57 32 14
> e-mail: [log in to unmask]
> website: www.netwing.at/nisters
> office:
> Natural History Department of the
> Tiroler Landesmuseum Ferdinandeum Innsbruck
> Feldstrasse 11a
> A-6020 Innsbruck / Austria / Europe
> phone: 0043 / 512 / 58 72 86 - 37
> e-mail: [log in to unmask]
> website: www.tiroler-landesmuseum.at