The Echinoid Directory

1a. Test composed of more than 20 columns of plates (more than 2 ambulacral or interambulacral columns in each zone)

1b. Test composed of just 20 columns of plates; interambulacral and ambulacral zones each composed of two columns of plates

2a. Test with bilateral symmetry superimposed over pentaradial plan; periproct displaced to the posterior - almost always lying outside the apical disc plates.

2b. Pentaradially symmetric; periproct opening surrounded by circlet of apical disc plates

3a. Perignathic girdle rudimentary or absent.

3b. Perignathic girdle well developed, composed of ambulacral auricles and/or interambulacral apophyses.

4a. Perignathic girdle always present and composed of interambulacral apophyses (ap) only. Ambulacral plating simple and extending onto the peristome.

4b. Perignathic girdle composed of ambulacral auricles (au). Ambulacra always compound.