Regular echinoids with:
Lower Permian to Recent; worldwide.
Cidaroids represent the sister group to all other extant echinoids (Euechinoidea). They differ from euechinoids by having a perignathic girdle composed of enlarged interambulacral elements (apophyses) and from all but the echinothurioids in having a series of ambulacral plates continuing over the peristomial membrane. The great majority have simple ambulacral plating, stout spines with external cortex and a single large primary tubercle dominating each interambulacral plate.