Hi Sue,
 
Although it is unlikely that a panther cowrie (Cypraea pantherina)
would be mixed in with a bunch of tiger cowries (Cypraea tigris) in a
gift shop, stranger things have happened.  A friend of mine found
seven beautiful specimens of a rare shell (Colubraria soverbii) in a
basket of otherwise common shells at a flea market.  The reason I say
it is unlikely is that most of the commercial grade shells that are
imported in bulk for gift shops and tourist places come from the
Philippines, and the panther cowrie isn't found anywhere near there.
Commercial shells are also imported from India, Taiwan, Australia and
East Africa - but the panther cowrie isn't found there either.  The
panther cowrie has a pretty restricted geographic range, mainly the
Red Sea and the waters just off the mouth of the Red Sea.  The tiger
cowrie on the other hand has a huge range, from South Africa through
the Indian Ocean and Red Sea, to northern Australia, most of the
Pacific Island groups, up to Japan, the Philippines and Hawaii.  In
other words, the tiger is found everywhere the panther is found, but
the panther is not found most of the places the tiger is found.  And,
the places where the panther is found don't have any large scale
commercial shell exportation that I know of.  Anyway, the presence of
a panther cowrie in a box of tiger cowries obviously wouldn't mean
they were collected together - it would just mean someone, somewhere,
put it in the box.  So, how can you separate the two species?  Even
though they are different enough that an experienced cowrie collector
can tell them apart at a glance, they are similar enough that they are
often confusing to less experienced collectors, since both species are
often about the same size (though the tiger does get bigger), and both
have large dark spots.  There are a couple of simple tests you can
try.
As you mentioned, the panther cowrie is a more slender species than
the tiger.  To be more specific, the length of a panther cowrie is
almost always more than 1.5 times the width, while the length of a
tiger cowrie is almost always less than 1.5 times the width.  So, if
you carefully measure the total length of the shell, and then its
width from side to side, at the widest point, and then divide the
length by the width, the resulting number will almost always separate
the two species.  If the number is greater than 1.50 it is Cypraea
pantherina; if less than 1.50, Cypraea tigris.  The measurements
should be taken with some kind of fairly accurate caliper - trying to
measure with a ruler may not give you sufficiently accurate
measurements.  This works for at least 99% of specimens.  I have seen
a couple of "skinny" tigris that squeaked over the 1.50 mark.  I have
never seen a pantherina "fat" enough to fall under 1.50 for
length-width ratio.  If you use 1.55 instead of 1.50 as the cutoff,
then 100% of the specimens I have measured are separable by this
criterion.
But, there is a simpler way - the ROCK 'N ROLL TECHNIQUE (the name
was applied by a clever 7th grade student at a presentation I gave
some years ago, and it fit so well I have used it ever since).  Place
the shell on a hard, flat, level surface - a tabletop or counter for
instance.  Hold the shell upside down, with the aperture (the
slit-like opening) facing up, and the smooth, rounded dorsum resting
on the tabletop.  Grasp the shell with your thumb on one side, and
your index or middle finger on the other side, keeping the base (the
surface now on top) approximately parallel to the table surface.
Then quickly open your fingers and release the shell.  If it ROCKS
but stays on its back, it is Cypraea tigris.  If it ROLLS over onto
its base (aperture down), it is Cypraea pantherina.  Who says
taxonomy can't be fun?  This method is also 99% accurate.  Every
pantherina I ever tried rolled over immediately, while ALMOST every
tigris remained on its back.  I have run into a couple of obstinate
tigris however, which seemed intent on mimicking pantherina in this
respect.  These are tigris with an unusually high dorsum.
 
Paul M.