try placing it on a piece of paper or something and taking a pic.
Anyways, at that size, possibly Planorbis arnoldii?
Here are two other threads where that snail came up:
Mine isn't flat from the top view its quite roundish looking. So I'm guessing its a baby ramshorn.Looks like a mini ramshorn. They are really flat like a disc and stay very small, like 2mm. I have them in most of my shrimp tanks and don't mind them at all. If you have a decent population of MTSs, the MTSs will outcompete them for food. Perfect dwarf puffer food.