I found this little snail under a log not too far from the Potomac River during last weekend's field trip. It's darker color in comparison to that of Zonitoides arboreus makes me think that it's Zonitoides nitidus, but I may be mistaken. A dissection would help me identify it more definitely, but I left the snail where I'd found it.
In Land Mollusca of North America (1946), Pilsbry wrote that the "animal" of Z. nitidus is black. He probably meant the snail's foot and that is why the shell, especially the body whorl, which is translucent, appears dark. The habitat of my specimen, near the Potomac, fits the habitat of Z. nitidus, which is, also according to Pilsbry, "generally found near water or in marshy places".