Several species of snails and slugs live around my house. One of these is the tiny Vertigo pygmaea that lives on and near the rocks that we use to build walls around flower beds. The average V. pygmaea shell in my backyard is 1.9 mm long. Because they are so small and have brown shells, they are hard to spot on rocks (and even harder on soil) unless you look very carefully.
The picture above shows 2 adults and a juvenile V. pygmaea that I found on the side of a large rock. You can see a close up of the 2 adults on the left. When the weather is dry these snails aestivate on the sides of or underneath the rocks. Sometimes you may see the same snail at the same spot for several days in a row.
Vertigo pygmaea is a widespread species. It is present throughout most of Europe and northeastern America. The genus itself is holarctic being present in North America, Europe and Asia. Pilsbry1 noted that fossil Vertigo had been recorded from the Eocene (~50 Myr ago) of Wyoming.
I have written about Vertigo gouldi, a close relative of V. pygmaea. The Vertigo species are distinguished from each other by the shapes and dimensions of their shells, the shell surface microsculpture and the number and positions of the teeth in the aperture. The picture below shows the teeth in the aperture of an empty V. pygmaea shell from my backyard.
1. Pilsbry, H. 1948. Land Mollusca of North America, II:2, p. 943.