The Echinoid Directory

1a. Periproct aboral and with subanal rostrum with ring-like subanal fasciole.

1b. No subanal rostrum. Periproct marginal or inframarginal.

Go to 5

2a. Anterior of test (towards top) extended out making the test vase-like.

2b. Anterior of test (towards left) not extended.

3a. Anterior (to right) broadened and bilobed so that test outline is triangular.

3b. Test cylindrical at anterior (to left).

4a. Plating in posterior ambulacra continuous.

4b. Posterior ambulacral plating disjunct; separated by plates of interambulacra 1 and 4 meeting along the midline.