|Channelled purple dogwinkle, Nucella canaliculata.|
The shell sculpture consists of many well-defined spiral ridges of approximately equal size. The whorls are set off from one another by a deep groove. Small individuals ... may be orange; older individuals are often yellowish brown or light gray. (WallaWalla.edu)
There is also another one, somewhat smaller, in the tank; it is bright orange.
Since there are two snails, I'll start watching for egg capsules and later, baby snails.
The famous purple dye from the city of Tyre that colored royal Roman robes, was made from a relative of Nucella. The snails were ground up in a stone mortar; different combinations made different shades of purple. The dye should be fixed with lemon juice as a mordant. The American species produce a much less brilliant purple than do the Mediterranean species. (WallaWalla)
I didn't know that. This shell would possibly produce a bluish-purplish grey. But the snail is safe; I'm not going to swipe his shell. When he's done with it, a few years from now, the hermit crabs will want it.