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Go to 2
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1a. Apical disc large; one or more periproctal plates tesselated around interior into ring creating an angular border.
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1b. Apical disc small with than peristome; hemicyclic or dicyclic always without periproctal plates tesselated into the disc.
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2a. Apical disc firmly bound to corona; ambulacral tubercles always much smaller than corresponding interambulacral tubercles.
Salenioida
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2b. Apical disc not bound to corona and usually missing; ambulacral and corresponding interambulacral tubercles approximately similar in size.
Phymosomatoida
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3a. Primary tubercles perforate
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3b. Primary tubercles imperforate and non-crenulate.
Go to 6
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4a. Interambulacral and ambulacral plates dominated by single large tubercle.
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4b. Interambulacral and ambulacral tubercles small and multiple [perforate and crenulate nature of tubercles often rudimentary at best].
Pedinopsids
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5a. Primary interambulacral tubercles much larger than ambulacral tubercles on aboral surface.
Hemicidaridae
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5b. Ambulacral zones not particularly narrow adapically; compound to apex
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Pseudodiadematidae
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6a. Primary tubercles crenulate.
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6b. Primary tubercles non-crenulate.
Go to 7
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7a. Ambulacral plate compounding diadematid with all elements reaching the perradius.
Stomechinidae
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7b. Ambulacral plate compounding arbaciid or stomopneustid with small occluded plate(s) bearing only pore-pairs.
Go to 8
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8a. Ambulacral plating trigeminate (rarely quadrigeminate).
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8b. Ambulacral plating with more than 5 elements to a compound plate.
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