Euechinoidea (class Echinoidea) Sub-class of sea-urchins in which the normally rigid test is composed of five ambulacra and five interambulacra, each made up of two columns of plates. The subclass includes both regular types of echinoid (e.g. Hemicidaris) and irregular types (e.g. Clypeus, Micraster). They first appeared in the Upper Triassic.

Euechinoidea (subphylum Echinozoa, class Echinoidea) A subclass of sea urchins in which the normally rigid test is composed of ambulacra and interambulacra, made up of two columns of plates. Spheridia are present. The subclass includes both regular and irregular types of echinoid. They first appeared in the Upper Triassic.