My favorite is the spirulid, or ram's horn squid. I was introduced to them in Australia, where I kept finding these perfect little white curls in the island beach wrack:
Based on the shell and some library books, I was able to identify it as the single living spirulid species, which bears the poetic name Spirula spirula. I was captivated by the pictures, but I've still never seen one alive.
The shell is internal, like the pen of a squid or the cuttlebone of a cuttlefish. And it's tiny--spirulids in their entirety are only a few inches long. They live in open tropical water all over the world, so it's the same species from Australia to the Canary Islands! They probably lay their eggs on the deep ocean floor. Beyond these bare bones (or should I say shells?), very little is known about them. With half an hour of reading, you can become on expert on this adorable creature:
©2004 Michael Vecchione
It's amazing and wonderful to me that there are still animals like this out there in the world.