These small snails are Zonitoides arboreus, commonly called the “Quick Gloss Snail” or “Orchid Snail.” This species is found in urban gardens and is native to North America. So, it looks like this is a California native living in our NHM Nature Garden! On Feb. 3, 2015, Jennifer Hudson, a visitor to the NHM with her two daughters found these specimens under the rocks and among the leaf litter in the Nature Garden.
Interestingly, while Zonitoides arboreus is native to most of North America, it has become an invasive pest in Hawaii, where it eats the roots of orchids in commercial orchid gardens. Morphologically, this species can be distinguished from species in the genus Oxychilus, by its smaller shell with obvious growth lines, higher spire, and more deeply impressed suture. See below: