They are egg cases from a juvenile thorny skate. 
The skate is related to sharks and rays. Sharks, skates, and rays all have a skeleton made up of cartilage, the flexible material that is found in our noses and ears. One tiny skate will hatch from each egg after nine months – hatching under the surface of the water. Usually, what we see wrapped up in the seaweed “wrack line” is the discarded egg cases. Another nickname for these egg cases is the “mermaid’s purse.” Check out this BCS post to learn the difference between skates and rays.
Image (c) NOAA – Alaska Fisheries Science Center.
Do you have another great question? Email info@beachchairscientist.com!
And don’t forget the Great Eggcase Hunt happens ever spring.
Oh yes, and then there is the classic beach chair riddle:
Q – What does a mermaid keep in her purse?
A- Her skates! 😛
We found some of these alive the other day in Oak Island NC. We brought them home from the beach in the hopes that we might see them hatch. We also thought that if we put them back in the surf, they would just wash up again. Obviously, this did not turn out well for the baby skates. They very quickly shriveled up and we no longer felt movement within the pod. Is there anything we could do differently next time? Are they just done for once they wash up on shore?
Just thought it might be worth mentioning that there is a small shark (sharks and skate are closely related) called the Small-spotted catfish (in Europe) who also lays these graceful egg pods. Theirs are more of a whitish tint. I use them in my artwork as a symbol for life’s grace and fragility. (www.studiogpg.com)