Two little urchins

On the right is a sea potato, a burrowing sea urchin that goes by the name of Echinocardium cordatum. Sea potatoes are plentiful in present day seas. I picked it up on our local beach.

On the left is a similar, but fossil urchin known as Holaster. It had a world-wide distribution back in the Jurassic-Cretaceous 195-70 million years ago.