The Echinoid Directory

Key to the major groups of Cidaroidea (1)

1a. Four columns of interambulacral plate in each zone at the ambitus.

1b. Two columns of interambulacral plate in each zone at the ambitus.

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2a. Two subequal primary tubercles on interambulacral plates.

2b. A single primary tubercle on interambulacral plates.

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3a. Both ambulacral and interambulacral plates with strongly imbricate sutures on aboral surface.
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3b. Interambulacral plates firmly tesselated together; ambulacral-interambulacral sutures either tesselate or imbricate.

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4a. Ambulacral plating biserial; pore-pairs uniserial.
4b. Ambulacral plating pseudocompound; pore-pairs biserial.

5a. Suture between ambulacral and interambulacral plates imbricate with adradial suture of interambulacral plates strongly denticulate all the way to the peristome (interior of interambulacrum shown).


5b. Suture between ambulacral and interambulacral plates tesselate, with adradial suture of interambulacral plates more or less vertical, except possibly in most adapical plates.

Cidaroida: Go to 6
6a. Ambulacral tuberculation heterogeneous at least adorally; every second tubercle enlarged.
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6b. Ambulacral tuberculation uniform throughout.

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7a. Primary interambulacral tubercles with massive mamelon; mamelons imperforate or with rudimentary perforation adorally.
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7b. Primary interambulacral tubercles always clearly perforate.

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8a. Ambulacral plates with cluster of subequal tubercles. Interambulacral plates without differentiated scrobicular ring. Primary interambulacral tubercle crenulate with massive imperforate mamelon.


8b. Ambulacral plates with marginal primary tubercle to each plate. Interambulacral plates with differentiated scrobicular ring surrounding areole. Primary interambulacral tubercle non-crenulate.


9a. Primary interambulacral tubercles strongly and obviously crenulate.

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9b. Primary interambulacral tubercles non-crenulate or with just traces of crenulation on aboral tubercles.


10a. Pore-pairs strongly conjugate.


10b. Pore pairs non-conjugate.

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11a. Primary interambulacral spines long and slender.


11b. Primary interambulacral spines stout and fusiform.

Plegiocidaris (stem group Cidaridae)