> >My taxonomic question deals with how do they determine rankings for any
group of similar organisms?  For instance, is the degree of similarity and
differences within the family Donacidae, the same as within the family
Turridae?  Or going outside of the mollusca, the insect family Carabidae,
or even the plant family Cucurbitacea?  Is it possible that a range of
differences that might warrent generic or subfamily ranking in one phylum
could be given family or superfamily ranking elsewhere?

Every year, firms have to produce a general accounting for the tax people.
In a big outfit, it is not always possible to account for everyone's
activities at all times, nor for where some of the stock is, so we have to
guess a bit and make some of it up for now. As long as we fill in all the
boxes with data which seems reasonable, the tax men are usually happy.
Smaller operations, however, can provide more accurate accounting, and have
to do a better job as anomalies are easier to spot.

Much the same is true of phylogeny. There are some very well-studied large
families within which the relationships are fairly clear, but these are the
exceptions. For most families - particularly among the bivalves - the
current structures are provisional and are likely to remain so for some
time. It could not be otherwise - if we understood these relationships well
enough to be able to produce an accurate and unimpeachable systematic
account of a massive and ancient phylum like the Mollusca, then we would
already know enough to clone humans, breed virus-specific antigens from
household garbage and do all sorts of other wonderful things. As it is, we
still have things like the Amathinidae, which as far as anyone can tell at
present contains genera which look like winkles alongside others which look
like limpets.

The current systematic structure is not immutable; for years, the Russians
had their own version, and even now some people use 'tribes' while others
don't. It's a convenience, nothing more, and the ranks are really just
arbitrary lines indicating perceived degrees of difference. As the
questioner surmised, these degrees are not the same in all families, and
are not based on a fixed number of common criteria.

In the end, a dog doesn't know it's a dog; still less does a snail know
it's a Partulid, or care for that matter.

PC.