I have tentatively identified this little charmer as Vallonia gracilicosta. A rather small snail, it is common to the Rocky Mountain regions, however, a few scattered populations exist east of the Mississippi (see map below). Scale ticks in photos are in mm.
Map of the distribution of fossil (tan) and living (blue) V. gracilicosta. The range for this species shifted from the Midwest to the Rockies after the retreat of the Laurentide Ice sheet 10,000 years ago. It is also possible that previous to the Pleistocene, V. gracilicosta inhabited the Rocky Mountain region, invading the midwest during the last glacial maximum. Map is from the US Census website, while biogeographic data comes from: Pilsbry, 1939; Nekola et al., 1999; Nekola, 2002; Frest and Dickson, 1986; Hubrict, 1985; Baker et al., 1986.
BAKER, R.G.; Rhodes, R.S., II; Schwert, D.P.; Ashworth, A.C.; Frest, T.J.; Hallberg, G.R.; Janssesns, J.A.; 1986, A full-glacial biota from southeastern Iowa, USA, Journal of Quaternary Science, Vol. 1(2), pp. 91-107.
FREST, T.J. and Dickson, J.R., 1986, Land snails (Pliestocene-Recent) of the Loess Hills: A preliminary survey, Proc. Iowa Acad. Sci. 93(3), pp. 130-157.
HUBRICHT, L. 1985, The distributions of the Native Land Mollusks of the Eastern united States. Fieldiana Zool., n.s. 24, vii+191pp.
NEKOLA, J.C., M. Barthel, P. Massart, and E. North. 1999. Terrestrial gastropod inventory of igneous outcrops in Northeastern Minnesota. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 60 pp.
NEKOLA, J.C. 2002. Distribution and Ecology of Terrestrial Gastropods in Northwestern Minneosta. Final Report: 2001-2002 Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 171+pp.