The Echinoid Directory

Key to families of Arbacioida

1a. Ambulacral plating without demiplates.

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1b. Ambulacral compound plates with demiplates.
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Eucosmus
2a. Ambulacral plating simple at ambitus; narrow; interambulacral zones dominated by a pair of large triangular plates adapically.

2b. Ambulacral plating trigeminate throughout; interambulacral plating standard.

Magnosia
3a. Ambulacral plating arbaciid with primary tubercle confined to central element and upper and lower elements reduced to demiplates.
Arbaciidae
3b. Ambulacral plating goniopygid with primary tubercle shared between two elements, the upper being distinctly smaller.
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4a. Distinct epistroma developed on aboral plates. Apical disc without perianal tubercles. Ambulacral and interambulacral tubercles similar in size.

Glypticidae

4b. No epistroma developed on aboral plates. Apical disc with perianal tubercles. Ambulacral tubercles generally much smaller than interambulacral tubercles.

Acropeltidae