Opeas pyrgula in the field. Left: 2 empty shells and 1 live adult (middle); Right: Juvenile.
Opeas pyrgula is considered to be originally a "tropical and subtropical" land snail species (Pilsbry, Land Mollusca of North America, 1946), but as a result of human activities, the species has long been distributed to many other parts of the world.
In Maryland, they are closely associated with railroads and may be found, sometimes in large numbers, under pieces of wood, large rocks or other types of semi-permanent material that provide a damp environment and offer protection from the weather. I suspect some railroad activity is responsible for their distribution, but I don't know what that may be.
I took the picture above in the field. The snails were on damp soil under a large chunk of concrete. On several occasions last winter at the same spot, I had found only empty shells; the live snails were present a few days ago. One wouldn't think that even a subtropical snail species would survive the winters this far up north, but they apparently do. It is unlikely that they are reintroduced every spring. I think O. pyrgula avoids the freezing surface temperatures by burying deep into the soil.
NatureServe lists records from several mostly eastern and southern U.S. states. There is even an unconfirmed record from Canada.
Like its close relative Subulina octona, another introduced species, O. pyrgula can also contort its body and shell into, what appear to my human eyes, awkward positions. The snail in the picture below was turning around on a horizontal surface.
More pictures of O. pyrgula are available at the Jacksonville Shell Club.