snails in the desert

When I think of deserts, I think of sand, heat, and aridity. Now, I know this isn't true for all deserts, but I think it's fair to describe the Sahara Desert in this way. What I don't think of when I think of deserts are snails - small molluscs who have a shell and an operculum (think of it as the front door of the snail's house that he can close as needed), but who also need to keep their muscular foot moist.
I was a little surprised to come across some snails from the Kurkur Oasis in Egypt here in the collection. Okay, so they're terrestrial (good because there's no ocean in the Sahara Desert) and they're from the Pleistocene (also good because there wasn't an ocean there at this point in time either), and they lived in an oasis (which by its very name suggests there is some moisture). But they're still snails. How do snails cross an 'ocean' of sand to find these oases? Darwin suggested passive dispersal that was probably carried out by birds, and this explanation has been fairly well accepted ever since.
More recently human movements and fishery activities have further spread certain organisms, but this is almost certainly not the case for these Pleistocene snails. At any rate, these snails made it into the desert and then back across the ocean to Connecticut.

 As described by the collector, Egbert Leigh, Jr., "this fauna bears a striking resemblance to a fauna in a small, nearly stagnant cove of Lake Carnegie, Princeton, New Jersey" (p.514, Desert and River in Nubia, Butzer and Hansen, 1968).
So, there you have it - stagnant pools in the middle of the desert.