Michael Reagin wants to know about modern calcitic gastropods. Can't help
you there, but Tertiary Ecphora is indeed calcitic, and so is Cretaceous
Striaticostatum, which looks much like Epitonium. Aragonite is harder than
calcite, but less resistant to solution. I don't know why a gastropod
lineage would switch from aragonite to calcite. Snails display a wide array
of shell structures, many of which have crystals laid down in different
directions to strengthen the shell, the way that wood grain at right angles
strengthens plywood.
 
Andrew K. Rindsberg
Geological Survey of Alabama