Cidaroida (Fossil Sea Urchin) Ring

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Cidaroida is an order of ancient sea urchins, ranking as the sole survivors of the subclass Perischoechinoidea. The other orders within this subclass, which were even more primitive than their living counterparts, met extinction during the Mesozoic era. Their primary spines are remarkably far apart compared to those found in other sea urchin species, and they lack gills altogether. Other key primitive characteristics include relatively simple plates within the test, as well as ambulacral plates extending continuously across the membrane surrounding the mouth.