unknown aquatic snail

I saw a few of these medium sized snails (and lots of empty shells) around small ponds near Dolores, CO. But one particular pond was full of them. The water was muddy but I could see scores, maybe hundreds of snails on the bottom. This snail came from that pond. Like many of the other snails, it had algae growing on its shell.

I hoped it would come out far enough for me to get a picture of its head, but didn't have the patience to wait more than a couple of minutes.