But it's not a snail. It's an isopod, specifically, a Philoscia muscorum*. Here it is before it was forcefully evicted from its abode, a shell of Cepaea nemoralis that was devoid of a snail.
Of the about 130 Cepaea shells I collected last weekend, 3 had isopods in them. Empty land snail shells are prime real estate for many invertebrates that are small enough to fit in them. I took that sentence verbatim from this post where the occupant of the shell was a thrips.
I am thinking of putting some empty snail shells in my yard to see if any isopods will move in.
*Thanks to Joan and Barbara for conforming the ID.