Hello Malia,

A dichotomous key that would allow identification of any shell at random is
simply not possible.  If such a thing existed, it would be the size of the
Encyclopedia Brittanica!  The first thing you would need to identify a shell
where the family is unknown would be a key to the families;  then, once the
family is identified, sub-keys within the family.  But, even a key that
could identify any unknown shell to family level would be impossibly
complex.

Shells actually are ideal specimens for teaching the use of dichotomous
keys, and I used them years back when I was teaching zoology at the high
school level.  I had a box of about 80 shells, from various families, which
I kept in the classroom permanently, and I had a dichotomous key which I
wrote myself, which would allow positive identification of those 80 shells
ONLY.  Once you write a key that is accurate for 80 species, you quickly
develop a sense of how complex such a key would have to be, in order to
include several hundred species or more.  My key to the 80 species was about
20 pages in length.  The students found the exercise interesting, and some
of them got pretty excited when they came up with the right name on their
own.

Alternatively, keys are available for some specific geographical areas, and
for some specific groups of animals.  For example, in one of my
undergraduate invertebrate zoology classes we used a dichotomous key to the
invertebrates of Cape Cod, which was published by the Woods Hole
Oceanographic Institute.  That key was a spiral-bound volume about an inch
thick, and probably covered a thousand or more species. It began with a key
to the phyla, then subkeys to the classes, orders, families, and species. It
was pretty complex, even for college science majors, simply because many of
the technical terms used were unfamiliar to anyone but the specialists who
wrote the various keys.

        Paul M.