Australia isn't big on freshwater snails. We don't have much in the way of permanent water bodies to support a great diversity of them. It should come as no surprise that freshwater snails tend to stick to the east coast, where seasonal rainfall keeps the rivers and wetlands filled. (At least, it did, back in the days when rain was a regular occurrence.)
Still, a few groups go nuts. On a straight number-of-species basis, almost all freshwater snails belong to Hydrobiidae. There are more species of hydrobiids than anything else. (Than everything else, I suspect. If only I could be bothered to add them up, I could give you a definitive answer. Such is life.) Hydrobiids are little apple pips of snails. I can barely spot them. And when I can, they all look the same too me.
So, let's ignore the hydrobiids in all their evolutionary exuberance and move on to something with a bit more substance to them.
The two species of Pseudopotamis are restricted to islands in the Torres Strait, close to the tip of Cape York Peninsula. Pseudopotamis semoni occurs only on Prince of Wales (Muralug) Island; P. supralirata on Hammond (Kiriri) Island. Despite the proximity to the peninsula (Prince of Wales Island is only about 20 km from the coast), no one has collected Pseudopotamis on the mainland.
That's an interesting distribution. The islands at the southern edge of Torres Strait are continental. They are the peaks of mountains that are spread out along an isthmus between Cape York and New Guinea. There's not much freshwater on the islands but there's plenty on the mainlands bordering the Strait. Why is Pseudopotamis so restricted? (Maybe it isn't. Maybe it's just been overlooked. Perhaps the streams on Cape York are jam-packed with them. Them and crocodiles.)
But there's another twist to this. According to Glaubrecht and von Rintelen (2003), the closest relative is Tylomelania. Tylomelania is endemic to the island of Sulawesi (Indonesia). Sulawesi lies in the heart of Wallacea, the wellspring of diversity between the land masses of Asia and Australia. A double-stranded necklace of islands and a couple of seas separate Sulawesi from Torres Strait. There's a Tylomelania- and Pseudopotamis-free gap of well over two thousand kilometres between them. Are we seeing the result of long-distance dispersal or an ancient vicariant event? Now there's a fascinating biogeographical story waiting for an author.
Glaubrecht, M. & von Rintelen, T. (2003). Systematics, molecular genetics and historical zoogeography of the viviparous freshwater gastropod Pseudopotamis
(Cerithioidea, Pachychilidae): a relic on the Torres Strait Islands, Australia. Zoologica Scripta 32: 415–435 (PDF)