Hi James,
There are a number of ways to rig things to hold the shell in place, if you
have the necessary graphics software to remove unwanted background from the
image.  If you don't have that, you could try laying a sheet of heavy black
paper over the shell (something like construction paper), then a magazine
over the paper.  That should hold the base of the shell flat against the
paper, and parallel to the glass of the scanner.
Paul M.