What Is It?

I found this partial jawbone while walking on the beach. I have no idea what manner of creature it belongs to.
What is it?
Why the hollows where teeth meet jaw?

It's a mystery
The bud-shaped teeth are growing two-deep in places.

My friend Ellen showed this to her father, who is a biologist, but he didn’t know what it was. His specialty is tunicates. While they do have mouths of a sort, they have no jaws. (Tunicates are distant relatives of all of us, our spinal columns having evolved from a structure similar to the tube that makes up a tunicate’s body.)

Any ideas?

P.S. Melissa just took lots of good knitting-related photos for me, so stay tuned.