Nerite snails

I am currently keeping two nerite snails (one zebra nerite, one with spikes on its shell) in my 10G community tank. It was about a month ago when I saw some nerite eggs on the aquarium glass. None of the eggs has hatched yet, but they are still on the aquarium glass. The eggs show totally no sign of fungus attack, and they are still perfectly white and oval. Is it even possible for the two different nerites to cross-breed at all? Or are they just unfertilized eggs? I still find eggs recently around my tank, especially on the bogwood, where my zebra nerite loves to rest on.

P.S. Some eggs are gone, but I don't find any little snails in my tank at all.

Any help would be greatly appriciated.

Unlimitedfishboy

I have another question which I forgot to ask on the last post:

I know there are some "Snailstrong" products which helps the snails grow better shells, but the price of it over here in Holland is extremely expensive. Are there any subsitutes which are cheaper? Or perhaps I can feed the snails with calcium-rich foods?

Any help would be greatly appriciated.

Unlimitedfishboy