The Echinoid Directory

1a. Aboral interambulacral plates imbricate.

1b. Aboral interambulacral plates firmly sutured.

2a. Primary spines slender, cylindrical and without thorns.

2b. Primary spines, stout and fusiform with densely scattered thorns.

3a. Interambulacral plates with confluent, ovate areoles more or less throughout. Spines with smooth cortex

3b. Interambulacral plates at ambitus and adapically with relatively separated, circular areoles; areoles confluent only adorally. Spines thorned or beaded

4a. Primary spines with dense thorned ribs.

4b. Primary spines with scattered coarse thorns along shaft.