Orders: Bothriocidaroida, Melonitoida, Cystocidaroida, Cidaroida, Diademoida, Holectypoida, Spatangoida, Clypeastroida.
- Ambulacrals generally compound, with two pores obliquely juxtaposed, in 2 columns as in all subsequent orders; interambulacrals usually with large radioles surrounded by smaller ones, as in Cidaroida, in 2 columns as in all subsequent orders, only one plate resorbed; mouth central, with jaws and external gills, sphaeridia present; anus aboral endocyclic. J.
It is, however, quite possible that Echinocystis may some day be referred to the Cidaroida, and Palaeodiscus to the Melonitoida.
This would leave the Echinoid scheme remarkably simple, with the Melonitoida and Cidaroida as divergent branches from an ancestor like Bothriocidaris; but while the former branch soon decayed, the latter continues to flourish at the present day.