Ceriths an Nassa snails

Do you mean Nassaurius when you say Nassa?
Well the best way to differenciate them is to flip them over and the one that can flip back is a Nassaurius as most of the algae/detritus eating snails are unable to turn around by themselves.
 
Nassarius snails have an elongated shaped foot while the cerith's foot is more compact. Nassarius have the "periscope", and are super speedy.
 
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