The septum resorbtion is another good way to check.  All neritaceans,
helicinaceans (terrestrial prosobranchs), and just to throw us off, all
ellobiids, do this.  Anybody have a couple extra Bathynerita to break?

I would need some convincing (not much) on the protoconch.  It's true,
neritaceans and helicinaceans have somewhat similar (in shape, usually not
sculpture) and recognizable protoconchs, particularly compared to naticids.
 It's a good avenue to explore.

Some people have used the radula.  It would be also be good in this case.
The differences are generally great between herbivores (neritids) and
carnivores (naticids).

Kurt