The Echinoid Directory

Key to the genera of Arbaciidae

1a. Aboral plates lacking tubercles, but with small glassy pustules that do not support spinelets or pedicellariae.
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1b. Ambulacral and interambulacral tubercles extend over aboral surface, although commonly absent from interradial zones.
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2a. Pustules small and irregularly scattered over interambulacral plates; not forming well defined rows.
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2b. Pustules forming a single horizontal row on each interambulacral plate.
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3a. Ambulacral plating simple towards peristome.

3b. Ambulacral plating compound throughout.
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4a. Single primary tubercle on each adoral interambulacral plate.
4b. Two small primary tubercles on each adoral interambulacral plate.
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5a. Pore-pairs above ambitus small and oblique.
5b. Pore-pairs above ambitus wide, horizontal and subconjugate.

6a. Basicoronal plate (in green) large, projecting interradially and bearing a large, obvious primary tubercle.


6b. Basicoronal plate small, not projecting interradially and without a large, obvious primary tubercle.

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7a. Vertical epistromal ridges link larger pustules in the interambulacral and ambulacral zones on aboral surface.

7b. Pustules not connected by epistromal ridges.