The Echinoid Directory

 Clypeasteroids with:

• internal buttressing in the form of concentric walls around the periphery with or without more central pillars;
• a pair of sunken and enclosed sphaeridia in each ambulacrum;
• petals when well developed consisting of alternating primary plates and demiplates (N. B. apetalloid forms with simple ambulacral plating);
• perignathic girdle of 5 pairs of ambulacral elements (auricles);
• interambulacral plating on oral surface always disjunct.

 Late Eocene to Recent; worldwide.