As stated and shown in another pic, the tell tale sign of a sundial snail is the cone shaped operculum. I love this image (thanks TG!)
I agree that your snails appear to be a type of nerite.
Here is a link with a good pic of the underside of a nerite
Nerites (Neritidae). You can see how the aperture in on the bottom. Another article by Shimek, which shows a diagram of the underside of the nerite, again, note the aperature:
The Grazing Snails, Part II - Abalones, Limpets and Nerites
Here are some more examples
Here is a link (or two
) to some thumbnails you can scroll through:
Neritidae Neritinae
Neritidae Theodoxinae
A question was asked about breeding. You will find egg masses sometimes from, for example, cerith snails (squiggles on the glass or rock), nassarius on the glass (clear egg case with dots inside), nerite eggs resemble sesame seeds, and my trochus mass spawn in the water column. The chances that they make it to actual adulthood is small. I did get baby trochus from my mass spawn, as there was no argument seeing the real tiny trochus (about 3) with red striping....which I watched get larger as time passed. There is also a mini turbo snail that alot of hobbyists think are baby turbos, but they are actually that small. I have them all over my tank at night.
Tiny Turbo. There is a possiblity that your snail is this, however, if I remember from your picture the shell has ridges? (insert Ruffles joke here). These tiny turbos (Macarene or Arene) appear to have a smooth surface to their shell.
Anyway, hope this helps. Another way to look at it, if you see them grazing on the rock surface or tank surface, then they are probably eating their food source (algaes). If they are eating a coral, then you've found their food source
.