Eduardo,

I have never had a problem scanning larger shells, so I can't help much.
However, I would recommend trying to scan at night with a dark room.  That
helps with the background and it may increase the contrast with the parts of
the shell further away from the glass.  It could be that your scanner just
doesn't have a bright enough bulb.  You might also try the software that
comes with the scanner to improve images -- I'll bet you have already tried
this, but just in case.  Most will lighten, darken, sharpen, smooth, etc.
and that might improve your image.  I have a Visioneer 6100 and it works
great.  It was also the cheapest scanner I found at the time I bought it
(and rated by Consumers as the best buy).

Tom Eichhorst in New Mexico, USA