I think it is an oversimplification to say that conchologists study
"just the shell" while malacologists study "the whole organism".  It
is certainly true that a conchologist concentrates on the shell.
However, a malacologist's research may focus on any aspect of
molluscan behavior, systematics, physiology, anatomy, or
biochemistry, and some such studies will inevitably concentrate on
the shell.  A conchologist, in my opinion, is primarily a hobbyist,
while a malacologist is first and foremost a scientist.  The term
"hobbyist" does not suggest a lack of in-depth knowledge or
expertise.  On the contrary, many advanced conchologists undoubtedly
know more about their subject matter - shells - than many
malacologists do.  However, their emphasis is not on scientific
research.  That, I think, is the dividing line between conchology and
malacology.  A researcher who is investigating the protein content or
trace elements or crystalline configuration or mechanical strength or
rate of dissolution of shells is certainly a malacologist, even though
he/she is studying "just the shell".
 
Paul M.