The Echinoid Directory

Key to the major stirodont clades

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1a. Apical disc large; one or more periproctal plates tesselated around interior into ring creating an angular border.
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.
2b. Apical disc not bound to corona and usually missing; ambulacral and corresponding interambulacral tubercles approximately similar in size.
3a. Primary tubercles perforate

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3b. Primary tubercles imperforate and non-crenulate.

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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].

5a. Primary interambulacral tubercles much larger than ambulacral tubercles on aboral surface.

5b. Ambulacral zones not particularly narrow adapically; compound to apex

6a. Primary tubercles crenulate.
6b. Primary tubercles non-crenulate.

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7a. Ambulacral plate compounding diadematid with all elements reaching the perradius.


7b. Ambulacral plate compounding arbaciid or stomopneustid with small occluded plate(s) bearing only pore-pairs.

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8a. Ambulacral plating trigeminate (rarely quadrigeminate).

8b. Ambulacral plating with more than 5 elements to a compound plate.