There are many orders of sea urchins. The ones in the Subclass
Euechinoidea go like this: Superorder Atelostomata - Orders
Cassiduloida and Spatangoida; Superorder Diadematacea - Orders
Diadematoida, Echinothurioida, and Pedinoida; Superorder Echinacea
- Orders Arbacioida, Echinoida, Phymosomatoida, Salenioida, and
Temnopleurioda; Superorder Gnathostomata - Orders Clypeasterioda
(sand dollars) and Holectypoida.