The Echinoid Directory

1a. Ambulacral zone with pore-pairs offset to left and right forming a biserial band.

1b. Ambulacral zone with pore-pairs arranged uniserially.

2a. Ambulacral zone with pore-pairs modestly offset to left and right and all elements reaching the adradial suture.

2a. Ambulacral zone with pore-pairs strongly offset to form two distinct columns and every other element almost excluded from the adradial suture.

3a. Interambulacral zones only about twice as wide as ambulacral zones; composed of four columns of plates only.

3b. Interambulacral zones very much wider than ambulacral zones; containing at least 8 columns of plates.

4a. Interambulacral zones composed of plates that vary somewhat in shape and size; no well defined en chevron rows.

4b. Interambulacral zones formed of very uniform rhomboidal plates, forming en chevron rows.

5a. Test very large; more than 30 columns of interambulacral plates at the ambitus in each zone.

5b. Test moderate in size; never more than 10-12 columns in an interambulacral zone.