I have one of those, I agree with pagojoe, and think it is Babylonia formosae. Mine usually stays under the sand but senses quickly when food is present and comes rushing out. Sometimes I can actually see its trunk (proboscis?) sticking up out of the sand, smelling for food, like a submerged submarine. It eats everything I feed my fish, and when it's fed it usually burrows down again. It moves very fast on the glass.
One peculiar behaviour I have seen is that it sometimes moves to the top of the aquarium and tries to raise itself out of the water. The shell weighs it down and it usually slips and falls to the bottom. Sometimes it does this repeatedly. I don't know why it tries to escape my tank, but I have seen pictures of other people's Babylonia do the same, so hopefully it's not something with my water.