Tetracidaris

Tetracidaris, a genus of Sea-urchins occurring in the Cretaceous rocks of France, which is of imporance as having more than two rows of plates in each of the five interambulacral areas. [For terms see ECHINOIDEA.] This character is otherwise on1y known in the Sea-urchins of the Palaeozoic period, which are separated largely on this character as a distinct group.