Most slugs in our gardens are introduced species. So, unfortunately, are most of the snails. That's right. Although you might only see the big Cantareus aspersus*, there are quite a few species of smaller snails that continue their activities in the mulch.
While I was weeding (an endless task in my garden), I found these two shells. They belong to Oxychilus, the glass snails of Europe, so named because the unworn shells are glossy and transparent. (Snail's Tales has a post on a Turkish species, O. urbanskii)
Three species have been recorded from Victorian gardens. They are difficult to tell apart, especially from worn shells. Even living specimens are tricky. Oxychilus cellarius and O. drapanaudi differ in size and in form of the body (last) whorl. Oxychilus alliarius smells of garlic when disturbed. Otherwise they're all very much the same, with those fine, featureless shells and blue-grey bodies.
These two individuals are probably O. cellarius, which is the largest of the introduced species. Like other Oxychilus, it feeds on soft material— invertebrates and decaying matter. Of course, that doesn't make it immune to attack from other predators. These might have fallen victim to other Oxychilus or, more likely, to snail-eating carabid beetles or flatworms.
*It used to be Helix aspersa