(subphylum Echinozoa, class Echinoidea)
A subclass of regular echinoids, without compound plates, in which the perignathic girdle is absent or of apophyses only, the teeth are grooved, and there are no gill slits or spheridia. The number of columns of plates in ambulacra and interambulacra is very variable except in Cidaroida. These echinoids first appeared in the Ordovician.
Subjects: Zoology and Animal Sciences.