I need to look it up again, but I believe that the Nassarius snails leave a tiny sack of eggs as where the Certh (I've watched the certh do this) snails leave the long zigzag strip of eggs. I've had both lay eggs in my tank but am only aware of the cerths actually surviving ( in abundance). It normally takes anywhere from a week to two weeks for the eggs to hatch if they are not consumed by another tankmate.