The Echinoid Directory
Key to the major clades of Irregularia Latrille
 |
 |
|
1a. Peristome relatively large, central and circular with buccal notches. Ambulacra with simple undifferentiated pore-pairs throughout |
1b. Peristome generally small and always lacking buccal notches. Ambulacra with differentiated tube-feet and pore-pairs (forming phyllodes and/or petals) |
 |
 |
|
2a. Plating of all five interambulacral zones on the oral surface similar |
2b. Posterior interambulacral zone on oral surface differentiated into a plastron |
 |
 |
|
3a. Single pore-pair to each ambulacral plate beneath petals. Pore-zones usually expanded close to peristome |
3a. Many pore-pairs to each ambulacral plate beneath petals. Pore-zones never expanded close to peristome |