The Echinoid Directory

1a. Apical disc ethmophract. Peripetalous fasciole passing vertically from plate 4 to plate 5 in interambulacral columns adjacent to the frontal ambulacrum.

1b. Apical disc ethmolytic. Peripetalous fasciole running across plates 4a and 4b in anterior interambulacra.

2a. Apical disc with two gonopores only (gonopores absent from anterior genital plates).

2b. Apical disc with three or four gonopores; the anterior sometimes reduced.

3a. Apetaloid; adapical ambulacral plates rudimentary.

3b. Petaloid.

4a.Petals deeply sunken and partially overarched so as to form only a narrow slit.

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4b. Petals modestly sunken at most and not overarched.

5a. Sunken part of anterior ambulacrum short; not extending anterior of the paired anterior petals. Posterior petals less than half the length of anterior petals.

5b. Sunken part of anterior ambulacrum long; extending to the anterior of the ends of the anterior petals. Posterior petals only a little shorter than anterior petals.