From my perspective, a malacologist does research on mollusks as
organisms. To put it another way, a malacologist is a zoologist
specializing in the phylum mollusca.  A conchologist studies/collects
shells.  Though the title malacologist is typically reserved for
professionals in the field, an amateur who investigates the biology,
natural history, systematics, physiology, anatomy, population
dynamics, biodiversity, etc. of mollusks is also working in the field
of malacology, and can often make significant contributions to the
body of knowledge in that field.  Such an individual, I feel, can
rightfully be described as an "amateur malacologist".  The term
amateur does not designate a person of little knowledge or experience
- it simply means a person who does not make his/her living in this
particular discipline, that is, a non-professional.  I think the
principal criterion separating malacologists from conchologists is
research.  One who collects shells, and reads/studies in an effort to
learn that which is already known and published about shells is a
hobbyist, and therefore a conchologist.  One who seeks to discover
new knowledge by original investigation is working in the capacity of
a scientist (whether professional or amateur), and is therefore a
malacologist.
 
Paul M.