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.
Go to 3
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3b. Interambulacral plates firmly tesselated together; ambulacral-interambulacral sutures either tesselate or imbricate. |
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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. |
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6b. Ambulacral tuberculation uniform throughout. |
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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. |
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9a. Primary interambulacral tubercles strongly and obviously crenulate. |
9b. Primary interambulacral tubercles non-crenulate or with just traces of crenulation on aboral tubercles. |
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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. |