Snails in the desert? How can that be? I've seen these before but this is the best, most perfectly formed shell I have ever come across. These are not fossils but recent inhabitants. They spend most of their lives resting under rocks waiting for the rains. They seal off their opening to prevent from drying out. I should add I have never seen a live one, only the empty shells.
This one is 5/8ths of an inch across. 12mm I think that converts to.