(class Echinoidea, subclass Perischoechinoidea)
An order of regular echinoids that have a radially symmetrical, subspherical test. The ambulacra are in two columns, with each plate bearing a single pore pair. The interambulacra are conspicuously wider than the ambulacra, each plate bearing one enlarged primary tubercle carrying a primary spine. The teeth are not keeled. There are no gill slits or spheridia. Cidaroida first appeared in the Late Silurian. There are six subfamilies, within two families, extant.
Subjects: Zoology and Animal Sciences.