#3814 - Flemingastrea wollastoni
A widespread genus of large extinct oyster, sometimes considered a subgenus of Ostrea, and named after British paleontologist and naturalist Andrew Fleming (1822–1892), who did a lot of work on the geology of India, which is just one of the places the oysters have been found. I don't know who Wollaston was. His oyster could reach 160mm in size.
This one was endemic to the shallow seas around New Zealand, but the various species were widespread around the world, from the Upper Cretaceous to the Eocene, and are quite useful index fossils, and markers for when sea levels rose to flood freshwater habitat. Although the way the shape and spikyness of the shells differs between young and old specimens doesn't help with ID.
Museum of the Vanished World, Duntroon, Aotearoa New Zealand.