The Turrid I posted on Ross's website (which by the way is Conopleura
striata Hinds) was scanned on an Epson 636U scanner, not an expensive
model.  However (OOPS) it was scanned at 7200 dpi, not 1200 as I
originally said (I was at work and relying on memory - now I'm at home
and have notes to consult).  I also have a Hewlett-Packard Scanjet IIC,
which is an older model, a real elephant compared to today's
featherweight (i.e. plastic) scanners, which also does a great job on
3-dimensional objects.
Resolution is of course important, but virtually all scanners these days
have more than enough resolution for scanning shells, unless you are
trying to scan the very smallest specimens.  The most important feature
to look for in a scanner for shell work is depth of field, that is, the
maximum distance above the glass at which an object can be defined with
a reasonable degree of sharpness.  Unfortunately, advertisements for
scanners rarely mention depth of field, because flatbed scanners are not
really designed to scan three dimensionally; so the manufacturers seem
to consider depth irrelevant - which in fact it is, for the flat
documents the device was intended to scan.  Some scanners, even some
very cheap ones, have considerable depth capacity, while others,
including some very expensive ones, have extremely little depth.  It
really seems that those models which do have good depth have it
inadvertently, rather than by design.  I don't know what gives a
particular scanner better depth than an other.  Of course, you only
image the lower half of the shell, so to get a good image of a shell 2
inches thick, you only need about one inch of depth of field.
My old HP scanner would scan with a stop-and-go motion, which caused the
device to vibrate or shake somewhat during a scan.  A smooth shell like
an olive or moon snail lying on the glass would often rotate a bit
during the scan, from these vibrations, so I had to resort to a variety
of means of anchoring the shell in place.  My Epson scans very smoothly,
and I think that is probably the case with the current generation of
scanners.  But it is still often necessary to use some sort of apparatus
to hold the shell in the particular position you want the image to show.
Another consideration in high resolution scanning is memory.  High
resolution images take up a LOT of memory.  The Turrid image referred to
above, as seen on Ross's site, is a 56K, 72 dpi JPG image.  However,
when it was first scanned as a 7200 dpi TIFF file, and while I was
processing it in Photoshop, the image was just over 30 megabytes. A
computer with 32 mb of RAM couldn't even open this image, much less
process it.  If I had scanned it at 1200 dpi, it would have been about 1 megabyte.
Regards,
Paul M.