The Echinoid Directory

 Stem group echinoids with:

• test composed of ambulacral and interambulacral plates; interambulacral plating strongly imbricate, ambulacral plating less so
• apical disc with 5 genital plates, each with an arc of multiple gonopores
• ambulacral zones greatly wider than interambulacral zones throughout; multiserial, composed of small hexagonal elements; pore-pairs small and undifferentiated throughout
• no internal perradial flange on ambulacral plates
• interambulacra composed of 3-5 columns of plates only.

Lower Carboniferous (Visean) to Permian, USA, Europe.

The great size of the ambulacral zones and corresponding reduction of interambulacral zones is seen in only one other group, the Proterocidaridae. Lepidesthidae differ from Proterocidaridae principally in having simple, uniform ambulacral pore-pairs; in Proterocidaridae there is a marked change in the ambulacral structure between oral and aboral surfaces and oral pore-pairs are much expanded and take the form of large circular structures.