A beautiful, huge, rather flattish heart urchin test found on the beach by my friend. I think it is Plagiobrissus grandis. She also found one with the spines on still (images 4 and 5). Many of the spines were very long, although lying flat.

(There is also another species off Nevis which is larger still, and much higher in profile, so it looks like a loaf of bread. That one is Meoma ventricosa I think, the Red Heart Urchin.)