I have a large sea urchin shell that the shop elf picked up oh so many years ago when he lived in Florida but I found an assortment on eBay that I ordered for a reasonable price. This allowed me to study the shape and patterns where the spines attach.
I am now going to make another one based upon the weird shell which is in the bottom row, first column and another regular shell in silver. Stay tuned for more pictures as I finish them!