Epitonids are calcite, and various muricids and buccinids may have varying
amounts of calcite as well.  I believe reported traces of calcite in
littorinids have not been confirmed.  Although I do not know of an example
from gastropods off the top of my head, calcite can occur in mollusks as an
abberation after shell or mantle damage or other environmentla factors, in
addition to the genetically-controlled occurrences mentioned so far.
 
David Campbell
 
"Old Seashells"
 
Department of Geosciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA
 
919-962-0685
FAX 919-966-4519
 
"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug