Cidaroida

Cidaroida, an order of Sea Urchins (Echinoidea) which contains the well known genus Cidaris, and a few closely allied genera. The characters of the group are that the ambulacra are narrow areas with the plates simple and not fused into compound ones; the spines and the plates (interambulacral) which bear them are both large: there are no external gills. The five arches which support the jaws are all incomplete. The genus Cidaris is the oldest surviving genus of Sea Urchins, as it commenced existence in Permo-Carboniferous times. The Chalk and Oolitic rocks yield many large species.