Took this photo at Hans and Kellie's house in Dee Why. It's a shark egg. They lay them near kelp beds and the eggs get lost and tangled up therein and so are protected from the eight gazillion things in the ocean that would love to eat them.
You can find the dried up and hard shells, if you're lucky, washed up on certain beaches. (Hans found it at a nearby beach.)
At the top you see the beginning of the long curly stringer that comes off the top of the egg - about seven inches long. It allows the egg to attach to the kelp. (Sorry or the lack of focus here.)
We didn't know what kind of shark it came from, but a little googling tells me it may be from a Crested Horn Shark. (Fact at bottom.) And this is cool.
Bonus: This one never hatched. You can feel the weight of the little shark embryo inside.