After watching all the 'what's a species' messages, I offer  the
following reference:
 
Mayden, R.L. 1997.  A hierarchy of species concepts: the denouement in
the saga of the species problem.  In: The Units of Biodiversity.  Edited
by M.F. Claridge, H.A. Dawah, and M.R. Wilson.  Chapman and Hall.
 
In it, he offers an account of at least 22 species concepts and how they
generally work.  Understand that he is analyzing these through a
systematics bias (species = independent unique lineages), but it's
interesting to read.
 
As for Liguus, try:
 
Hillis et al. 1991.  Minimal genetic variation in a morphologically
diverse species (Florida tree snail Liguus fasciatus).  Journal of
Heredity 82:282-286.
 
Again, what you get out of it depends on how you view species and such.
Just thought I'd offer some refs.
 
Russ