This snail didn't have the reflected adult lip yet. (Shell diameter=1.5 mm)
I found this
I don't think I had recorded V. costata in my backyard before. It probably came with the soil or rocks that we frequently bring to our yard from elsewhere.
This species has a widespread distribution. According to Jochen Gerber*, it has been recorded from most of Europe, Russia, Armenia, Turkey, Afghanistan, Uzbekistan and the U.S. There are also records from Algeria and South Africa. It may have been introduced to some of those countries, but I believe it is thought to be native to the U.S.
Note added 10 January 2008: Earlier this week Jochen Gerber of the Field Museum in Chicago contacted me by e-mail to let me know that he didn't think that this snail was Vallonia costata. He explained that both the color of the shell ("too brown") and the snail's body ("quite heavily pigmented") did not agree with the usual colors of Vallonia shells and soft parts. I hadn't paid attention, especially, to the body pigmentation. After I did a quick literature check, I realized that Jochen was, of course, right. Pilsbry (Land Mollusca of North America, 1948) says "soft parts are transparent white" and the photo of V. costata in Fechter & Falkner (Weichtiere, 1990) shows a snail with an entirely whitish body. Jochen also suggested that this snail could instead be a Paralaoma sp. A more accurate identification will have to wait for me to find the snail again, because I didn't keep it the first time, but returned it to my yard.
*Gerber, J. 1996. Revision der Gattung Vallonia Risso 1826. Schriften zur Malakozoologie 8:1-227.