The Echinoid Directory

Key to the major clades of Palaeozoic echinoid (2)

6a. All but the most adapical interambulacral plates with large tubercle surrounded by a clearly defined and typically sunken areole.

6b. Interambulacral plates lacking a large primary tubercle, or with such tubercles confined to only the most adradial series.

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7a. Ambulacra wide, much wider than interambulacra; composed of multiple columns of hexagonal plates.

7b. Ambulacra relatively narrow, significantly narrower than interambulacra; biserial or quadriserial at most.

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8a. Ambulacral plates each with a circular area of 6-12 pores surrounded by a periporal rim.

8b. Ambulacral pore-pairs of standard structure with two pores.

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9a. Ambulacral pore zones quadriserial.
9b. Ambulacral pore zones uniserial.
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10a. Interambulacral plates rhomboidal and more or less regularly arranged into horizontal and vertical rows; interambulacral plates finely tuberculate.


10b. Interambulacral plates not forming well defined rows and columns; interambulacral plates without tubercles.