One additional thought, within the context of what I said before - there
probably is a reason why the smallest protoconchs are not found in land
snails.  Among marine snails, the smallest protoconchs (and sharpest apices)
are generally found among those species which have a free-swimming veliger
(embryonic) stage in their life cycle.  Species which give birth to fully
formed, shelled young, or in which shelled young emerge from an egg capsule,
usually have a larger protoconch.  Since land snails do not have a veliger
stage, their minimum protoconch size is likely to be larger than that of
aquatic species.
Paul M.