The Echinoid Directory

Key to the major clades of stem group Euechinoidea

1a. Interambulacral zones composed of just four plates, one adoral and three aboral.

1b. Interambulacral zones with at least 10 plates arranged biserially.

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2a. Primary interambulacral tubercles non-crenulate and either imperforate or with a rudimentary perforation.

2b. Primary interambulacral tubercles perforate and crenulate.

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3a. Ambulacral plating pseudocompound; pore-pairs uniserial apically, biserial adorally; tuberculation heterogeneous.

3b. Ambulacral plating simple; pore-pairs uniserial throughout; tuberculation uniform.