The Echinoid Directory

1a. Plastron composed of wedge-shaped plates becoming biserial towards posterior.

1b. Plastron composed of uniserially-stacked polygonal plates.

2a. Anterior genital plates separate. Apical disc generally subcentral. Frontal groove feeble or absent.

2b. Anterior genital plates fused. Apical disc always close to anterior border. Frontal groove well defined.

Go to 3

3a. Plastron plating disjunct. Apical portion of test extended as a slender rostrum.

Hagenowia

3b. Plastron plating contiguous. Apical portion of test acute but not prolonged.