The Echinoid Directory

1a. Scrobicular tubercles absent from adambulacral margin of interambulacral plates.

1b.Scrobicular tubercles forming a complete circle around interambulacral tubercle.

2a. Apical disc rounded to pentagonal in outline; ocular plates not projecting.

2b. Apical disc with square-ended, projecting ocular plates.

3a. Apical disc raised and cap-like; smooth and unornamented other than by sutural pits.

3b. Apical disc more or less in flush contact with remainder of the test; ornamented by coarse pustules; sutural pitting never developed.

4a. Ambulacra extremely narrow, with no granulation between double column of primary tubercles.

5a. Ambulacra bigeminate adorally and/or throughout (one tubercle to two pore-pairs).

5b. Ambulacra unigeminate throughout (one tubercle to each pore-pair).

6a. Ambulacral plating simple except immediately adjacent to the peristome where it becomes bigeminate.