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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.
Tiarechinidae

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.
Triadocidaridae

2b. Primary interambulacral tubercles perforate and crenulate.

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

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

Paurocidaridae