It is a bivalve shell cast
Pretty much.
It's an internal cast of the "Cucullaea gigantea" extinct giant clam, or something very closely related. I've collected plenty of them.
=============================================================I don't think it's a tooth. Looks like a bivalve shell to me. Like a muscle or clam. Where did u find it? Those teeth don't just turn anywhere and are specific to certain geologic formations.