Ambulacral plating simple or incipient pseudocompound at most; Perignathic girdle absent, or with feeble muscle attachment points at median and adradial edges of interambulacral zones. Kier (1984) ...

This is a synapomorphy for the crown group of the Echinoidea, and is also true for all post-Palaeozoic echinoids, bar one, Tiarechinus. By contrast almost all Palaeozoic echinoids have a test which ...