Murex Trapa, or Rare-spined Murex
According to a fact sheet on wildsingapore, the Murex Trapa is collected as food by coastal dwellers, and its strikingly beautiful shell is also coveted in the shell trade. It is also listed as Vulnerable and "seldom seen" today, because it is an intertidal creature that is easily affected by reclamation and over-collecting. These snails do not live in the Singapore River, so someone must have been collecting the shells from the intertidal zone and bringing them in, for there are so many of them!
[Thank you so much to Tan Swee Hee from the Raffles Biodiversity Museum for showing me the collections of the shells from Pulau Saigon and the other rock cores, meteorites, and other geological oddities from around Singapore!]
Pyrite can be used for a variety of uses such as for the production of sulfuric acid, or for uses in the paper industry. What could these have been used for in Singapore? It is not clear to me, perhaps it will take a closer study of the specific industries that used to be on Pulau Saigon to determine this. Pyrites are also used in fengshui, where they are considered to be a good fengshui stone to attract abundance. When I was young, I had a collection of semi-precious rocks and my collection included pyrites, or Fool's Gold. So I also instinctively think of pyrites as being a kind of "collector's item", having collected it before as well. So no matter what it might have been used for, it could also very well have been part of someone's collection of rocks...
For more on Pulau Saigon, see:
The Chert of Pulau Saigon, a former island in the Singapore River
Bone, Metal, Wood, and Other Artefacts found on Pulau Saigon