Lymnaea humilis? Seems to be some differences between sources in size and shape of aperture, though it also seems as though several species descriptions have been declared one species. Lymnaea humilis. It fits the description of Fossaria parva in The Freshwater Snails of New York State which turns out to be a synonym of Lymnaea humilis. Seems to be variation in size (6 to 15mm) and relative shape of aperture. Found just at the waters surface on a rock.