Hi Ellen,

Yes, suspending the shell in a liquid sounds right.  Otherwise the spines
will inevitably get chipped when the vial knocks against something.  It
would have to be a fairly viscous liquid in order to provide such
protection, and obviously something that will not react with the shell.
Mineral oil sounds like a good bet.  It also has good optical properties, so
will allow clear viewing of the shell without distortion (of course, the
curvature of the glass will cause some distortion, but that can't be helped
if you are going with a round vial (perhaps a flat-sided vial - square or
traingular - would be an option?).
Whether the shell will float due to entrapped air is uncertain.  A
lightweight shell like a small Cypraea, Melampus, or bubble shell would
definitely float in mineral oil unless interior air was removed.  A solid
shell like a Drupa or Morula definitely would not float, even if filled with
air.  A shell of intermediate weight  and limited interior space - like a
Typhis - could go either way.  Interior air can be removed from a small
shell by exposure to vacuum, as described in a recent post on cleaning
shells.

Paul M.