Canas??

I bought 8 snails today from a guy in Pearland for $30. He kept them in a pond in his yard and a couple in his aquariums. I think they are Canas, but I'm not quite sure. He said they get as big as baseballs and they voraciously eat any plants so that leads me to think that's what they are. We couldn't find any that large since it was the middle of the day and most were in the middle of the pond. He said he'll try to pick one of the huge ones out for me and give me a call. They are mostly brown with darker stripes, but one I picked out looks gold with stripes. They are all covered with algae, so it's hard to see their true coloring.