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1a. Ambulacral plating without demiplates.
Go to 2
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1b. Ambulacral compound plates with demiplates. - Go to 3
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- 2a. Ambulacral plating simple at ambitus; narrow; interambulacral zones dominated by a pair of large triangular plates adapically.
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- Eucosmus
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2b. Ambulacral plating trigeminate throughout; interambulacral plating standard.
- Magnosia
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3a. Ambulacral plating arbaciid with primary tubercle confined to central element and upper and lower elements reduced to demiplates. - Arbaciidae
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3b. Ambulacral plating goniopygid with primary tubercle shared between two elements, the upper being distinctly smaller. - Go to 4
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4a. Distinct epistroma developed on aboral plates. Apical disc without perianal tubercles. Ambulacral and interambulacral tubercles similar in size.
- Glypticidae
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4b. No epistroma developed on aboral plates. Apical disc with perianal tubercles. Ambulacral tubercles generally much smaller than interambulacral tubercles.
- Acropeltidae
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