The Echinoid Directory

Key to the genera of Triadocidaridae

1a. Apical disc small (ca. 1/3 test diameter) and stellate, ocular plates projecting well beyond ring of genital plates, so that ambulacral zones start a little removed from the ring; dicyclic.
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1b. Apical disc large subpentagonal opening at least half the test diameter; Ocular plates not projecting beyond genital ring, presumed monocyclic.

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2a. Tubercles imperforate; ocular plates in contact with genital plate ring.

2b. Ambital and adapical tubercles with rudimentary perforation; ocular plates embedded within interambulacra and separated from genital plate ring.


3a. Interambulacral tubercles with clearly visible non-crenulate platform surrounding the mamelon; no scrobicular ring.


3b. Interambulacral tubercles with massive mamelon so that there is no surrounding platform; scrobicular circle usually distinct.

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4a. Interambulacral tubercles perforate more or less throughout.


4b. Interambulacral tubercles imperforate or with just a rudimentary perforation on adapical mamelons.

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5a. Approximately 3 ambulacral plates opposite an interambulacral plate at the ambitus.


5b. At least 6 ambulacral plates opposite an interambulacral plate at the ambitus.

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6a. No tuberculation outside scrobicular circles.


6b. Interradial zones wide and densely tuberculate.