How do you get the fourth Trilobite key in Mardek 2

The trilobite keys are found in the following places
1. The tainted grotto, there is a tresure chest on the map but it is actually in one of the green pools
2. Kill zombies Canoia until you get one
3. Found in one of the coffins in Social Fox's tomb marked on the map as a tresure chest, Social Fox's tomb is located before where you fought Moric
4. This one is only available if you spare Gope's life, go to Goznor and talk to him in the inn