The Echinoid Directory

1a. Ambulacral zones biserial throughout; pore-pairs either uniserial or weakly offset to left and right.

1b. Ambulacral zone composed of more than two columns of plates; pore-pairs forming broad adradial bands or multiple discrete columns.

2a. Pore-pairs relatively large and central, leaving little perradial zone.

2b. Pore-pairs small and distinctly adradial in position, with a modest perradial zone.

3a. Ambulacral plating more or less uniform.

3b. Every second ambulacral plate thinning towards adradial suture and almost excluded.

4a. Ambulacral zones quadriserial throughout .

4b. Ambulacral zones with more than four columns of plates in each half-ambulacrum; pore-pairs forming broad adradial bands.

5a. Outer series of ambulacral plates almost as wide as inner series so that the ambulacra have four widely separated columns of pore-pairs; additional occluded plates between inner and outer series.

5b. Outer series of narrow demiplates and inner series of much wider plates; pore-pairs forming single adradial band of two closely spaced columns.