The Echinoid Directory

Irregular echinoids with:

• test bilaterally symmetrical;
• apical disc compact, tetrabasal or monobasal;
• petals developed (secondarily reduced in some taxa); all five ambulacra generally similar and flush;
• peristome small, without buccal notches;
• periproct to posterior of genital plates 1 and 5; usually separated from apical disc but fully enclosed in many primitive members;
• spines small, uniform and dense; hollow.

Lower Jurassic (Toarcian) to Recent.

Distinguished from Holectypoida by the lack of buccal notches and the presence of petals. The other major group of irregular echinoids, the Atelostomata, primitively have an elongate apical disc. Only Spatangoida have a compact apical disc like the Neognathostomata, but they have interambulacrum 5 differentiated as a plastron.