Thanks for your helpful explanation re keys, Paul.
I was convinced that I had found a common sand-dwelling Terebra on my
recent visit to India, just judging by the shell. But now I think it is
more likely to be Bullia tranquebarica. If I only looked at examples from
the Bullia genus, I wouldn't have guessed that either without seeing the
photo of the more Terebra-like B. tranquebarica. So a key based on shell
morphology would probably not have helped in that case, right?
David
Durham, NC


>A simple key or guide to the families of marine shells sounds like a good
>thing, but writing one would be far from a simple project.  The way an
>identification key works of course, is by gradually excluding more and more
>groups, through the selection of increasingly similar characteristics, until
>all but the correct group have been eliminated.  The problem is, some
>families are so large and diverse that there are virtually no conchological
>characteristics common to all members of the family.  In Trochidae for
>example, there are tall slender species that look like auger shells, and
>flat ones that look like sundial shells; smooth, glossy ones and coarsely
>sculptured ones; thin and translucent vs. thick and heavy; open umbilicus
>vs. closed; toothed apertures vs. toothless; nacreous vs. non-nacreous (both
>inside and out); etc.   About the only shell features that all trochids have
>in common are the presence of an apex and an aperture, which is hardly
>sufficient to separate them, en masse, from other families.  Therefore, to
>write a key that would simply allow a person to identify an unknown shell as
>a Trochid would mean writing at least dozens, if not a hundred or more
>separate branches, or sub-keys, taking into consideration all the diversity
>of the family.  And even if a person took the time and did the research to
>accomplish that, it would not be foolproof.  Nora pointed out the difficulty
>in separating some members of Trochidae from Turbinidae.  Indeed, some of
>them are so similar that a key based on shell structure alone could not
>separate them.  The only way I can separate Astralium haematragum
>(Turbinidae) from Trochus saccellum (Trochidae) is by the operculum.  If the
>specimen doesn't have an operc, it could be either species, and therefore
>either family.  Some other families, like Modulidae (and even some land
>snail families!) include species that are very similar to Trochids.
>One thing we need to keep in mind as shellers is that a shell is just a
>piece of something, in fact a relatively simple component of a much more
>complex organism.  Even though we commonly speak of "new shells" being
>described and named, we know that it is really new species of animals that
>are being discussed, not just their exoskeletons.  Often the assignment of a
>species to a family is based largely on non-conchological characteristics.
>And, we know that classifications based on the shell alone sometimes have to
>be revised when the whole animal is eventually studied.  The best-known
>example of this is probably Marginella (Afrivoluta) pringlei, the world's
>largest margin shell, which was classified for many years as a volute, based
>on the shell.  A key to families would actually be much easier to write if
>ALL morphological and anatomical characteristics were considered.  The
>problem is, once written, such a key would be impossible for the average
>person to use, since we typically don't have access to the radula,
>periostracum, tentacles, foot, eyes, reproductive organs, etc. of our
>specimens.
>Years ago, when I was teaching a high school zoology course, I put together
>a box of about fifty assorted shells, and wrote a dichotomous key that would
>identify, to species level,  just that particular group of shells.  This was
>simply to let the kids get a feel for how a key works.  Even in that modest
>venture, I found that some specimens had to be identified through more than
>one branch of the key.  This was because a characteristic which would
>clearly separate most of the shells into two subgroups might be ambiguous
>for a few specimens.  Therefore, in order to ensure the correct
>identification of those specimens, they had to be included in both branches.
>It took me some hours to come up with a workable version.  (I know this must
>be confusing for anyone who isn't familiar with such keys and how they work
>- sorry).
>Paul M.