Many snails have an operculum, which is a sort of “door” attached to the snail’s foot. When the snail retreats into its shell, this door covers the shell’s opening, protecting the animal against predation or dessication. Depending on the species, the operculum may cover the entire opening or just part of it. Also, depending on the species, the operculum may be leathery in appearance and somewhat pliable (“corneous”), or it may be stony, and made of calcium carbonate like the shell itself (“calcareous”). The leathery ones dry out and lose their luster when exposed to air. The one pictured had just washed ashore and I was able to take the photo within an hour of finding it. What luck!