The Echinoid Directory

1a. Interambulacral plates with a single primary tubercle, although smaller secondaries may occur both adradially and interradially.

1b.Interambulacral plates with a row of several subequal tubercles. Areoles forming distinct platforms.

2a. Spines verticillate.

2b. Spines non-verticillate, with very fine granular ornament irregularly scattered over the shaft surface.

2a.Ambulacral plates with small primary tubercle and 4-6 large secondary tubercles almost as large.

2b.Ambulacral plates with a single large primary tubercle.

3a.Pore-pairs expanded adorally to form a phyllode; no adradial band of tubercles on ambulacral plates. Test plating firm.

3b.Pore-pairs remain uniserial adorally and shift to the centre of the plate. There is a wide adradial zone with tubercles almost as large as the perradial primaries. Test plating somewhat flexible.

4a.Aboral surface with well-developed tubercles.

4b.Aboral surface largely naked.