The Echinoid Directory

1a. Test composed entirely of ambulacral plates, with a perradial column of imperforate plates flanked by one or more columns of perforate plates. Plates always thick and sutures vertical.

1b. Test with imperforate series of interambulacral plates separating ambulacral zones. Plates commonly thin and imbricate, though they can be thick and tesselate.

2a. Test with perradial column of imperforate plates in addition to paired perforate ambulacral plate columns and paired interambulacral plate columns.

2b. Test composed of alternating zones of perforate ambulacral plate columns and imperforate interambulacral plate columns.

3a. Single column of interambulacral plates alternates with ambulacral zones.

3b. At least three columns and up to 30 columns of interambulacral plates in each zone.

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4a. Marked difference in ambulacral structure and pore morphology between oral and aboral surfaces. Oral pore-pairs enlarged circular structures with prominent peripodial ring.

4b. Ambulacra not expanded adorally and pore pairs small and undifferentiated, morphologically identical on oral and aboral surfaces.

5a. Interambulacral plates thick and hexagonal, with vertical sutures and with a fine uniform covering of granules.

5b . Interambulacral plates relatively thin and clearly imbricate with oblique sutures; almost always bearing distinct tubercles.