Sharon:

A couple of years ago I bought a dissecting microscope at Radio Shack for
about $150.  It comes with interchangeable optics of 10X, 20X and 30X.  I
find that I almost always use the 20X for shells as small as 1 mm. and less
up to 10-15mm. 30X is good for more detail, but of course along with that
comes loss of depth of field.  There is no light source with this microscope,
so I bought a couple of small, flexible desk lamps, and with 60 W. bulbs they
work fine.  After two years of use, I have no complaints at all about the
quality of the microscope, and it cost several hundred dollars less than
anything I found in various scientific catalogs.

Good luck,

Herb Chapin