Dear Marlo;-
        If you would like us to say that a "microshell" is any adult shell
under 4mm or3mm, we will be happy to acommodate you. But it just doesn't
do. There is NO solid millimeter description or limit. The concept of
"microshell" is simply that----a concept. I think of a "micro" as any
shell that can only be distinguished by a three or four power lens, and
can (the whole of it) be seen under such a lens.
        I am sure that whatever distinction you make will be just as good as
mine.
        Art