Back in the 1930s, the little* Pomatiopsis lapidaria was the subject of a series of heated exchanges between 2 of the leading malacologists of the period, Frank C. Baker and Bohumil Shimek. Baker wrote that P. lapidaria was an amphibious snail, while Shimek argued that it was terrestrial (see the back to back salvos of Baker & Shimek at each other in Ecology, vol. 11, pages 788-791, 1930).
The controversy was never resolved. Pilsbry did not include P. lapidaria in his Land Mollusca of North America (1940-1948), but Hubricht did in his The distributions of the native land mollusks of eastern United States (1985).
During our field trip to LaRue Pine Hills in Illinois last month, we found P. lapidaria on a steep hillside away from any running or standing water and in the company of unambiguous land snails. But, apparently, it can also survive for considerable periods in water, which is not surprising, because it still maintains a vestigial gill (ctenidium) in its mantle cavity.
Pomatiopsis lapidaria is an operculated snail in the family Pomatiopsidae, superfamily Rissooidea. There are 2 other families in the Rissooidea the representatives of which have been on this blog more than once: Assiminea succinea and the genus Truncatella. Whether they are aquatic or terrestrial can also be argued endlessly. Obviously, the root of the "problem" is with the Rissooidea: they have so far evolved just enough to get out of the water, but not to survive far from it.
More or less everything we currently know about P. lapidaria is in Dundee, D.S. Aspects of the biology of Pomatiopsis lapidaria...Misc. Pub. Mus. Zool. U. Michigan #100, 1957 (pdf).
*Adults grow to be about 6-7 mm in shell length.