Hello, I find these occasionally on the beaches of southern California. I am really not sure of what they are. If anyone can help me solve this mystery I sure would appreciate it. thank you
They look like pieces of a bigger shell to me, but worn by the sea. I don't think they are erchins or such, but hopefully someone with more knowledge of the areas species will prove me wrong.
Best wishes,
Emma.
thank you for your response
 I have a whole collection of these and they all seem to have the same pattern/ridges.