It's the "skull" of a shark. Sharks have no bones so, technically, don't have skulls. (A shark's skeleton is made of cartilage, not bone.) Therefore, the "skull" of a shark is properly known as its chondrocranium.
Oh, and
sharks do get cancer, just like all other vertebrate animals. While it's true that cartilage is a lot less prone to become cancerous than are many other body tissues, even bone, that's because cartilage has no direct blood supply and therefore grows very slowly as a rule. Slow-growing tissues with no direct blood supply rarely become cancerous. So taking "shark cartilage" pills will not reduce your chances of getting cancer. All it will do is make you a little poorer, some quack a little richer, and some poor sharks dead.