Hi Ross,
 
You wrote:  <Mytilus trossuls Gould, for instance, overlaps so thouroughly
with M.
edulis L., that only an arcane set of ratios can tell them apart almost 100%
of the time (they are so-called "crypto-species")>
 
Serious question - if two species are so similar that it takes mathematical
computations to separate them -  (1) how were they recognized as distinct in
the first place; and (2) how do we know they really are distinct?
 
A classic example of what you call "crypto-species" (as I understand your
description) involves two shells in completely different families - Trochus
sacellum rota Dunker (family Trochidae) and Astraea (Astralium) haematraga
Menke.  The shells of these two species are virtually indistinguishable -
unless you have the operculum.  The Trochus of course has a corneous
operculum, while the Astralium has a calcareous operculum.  But if it were not
for that difference, I cannot imagine how the two would ever have been
recognized as separate species, let alone separate families.  Do you have any
information on how these two mussels were recognized as separate species?
 
Paul M.