The Echinoid Directory

1a. Sternal plates asymmetric; median suture obviously oblique.

1b. Sternal plates symmetrical; median suture more or less sagittal.

2a. Anterior and posterior petals large and subequal in length.

2b. Posterior petals distinctly shorter than anterior petals.

3a. Anterior petals only about twice as long as posterior petals. Peripetalous fasciole restricted to a single interambulacral plate in each column.

3b. Anterior petals at least three times longer than posterior petals. Peripetalous fasciole indented by one or more plates behind anterior petals.

4a. Peripetalous fasciole incomplete; present only around the posterior part of the test.

4b. Peripetalous fasciole more or less complete.

5a. Two gonopores (both anterior gonopores missing).

5b. Three or four gonopores.