I'd be willing to bet they were Viviparus malleatus, or a similar Viviparus species. These are eaten in some parts of the world, they are large enough to be worth the effort (up to 50 mm or so), and they thrive in muddy, weedy, stagnant water like park ponds. I have been to similar ponds in the northeast where you could stand on the shore and see at least a couple of dozen snails. If it's a sparkling clean pond, you won't see them. But if its a trash-filled, stagnant mudhole, they love it! Paul M.