The Echinoid Directory

Key to the genera of Trigonocidaridae

1a. Apical disc with single tubercle on inner edge of othewise smooth genital plates.
1b. Apical disc tuberculate, usually with a ring of tubercles surrounding the periproct.
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2a. Test without any obvious pitting or sculpting.
2a. Test with pitting and/or epistromal sculpting of the test.

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3a. Secondary tubercles on coronal plates dense and uniform; ornament confined to small radial bars and pits around the primary tubercle.
3b. Secondary tubercles sparse and heterogeneous, intersperced with epistromal ridges. Radial ornament around primary tubercles.

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4a. Marked reduction in size of primary tubercles above the ambitus; epistromal ornament with strong vertical component linking tubercles dominates aboral plates.


4b. Primary tubercles not markedly smaller above ambitus; aboral plates not dominated by coarse epistromal ornament.

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5a. Pore zones offset in triads.

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5b. Pore zones uniserial or in weak arcs throughout.

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6a. Epistroma in the form of zig-zag vertical ribs and grooves.


6b. Epistroma in the form of reticulate ribs and grooves surrounding primary tubercle.

7a. Ambulacral plates with primary tubercles linked by zig-zag perradial ribs; no secondary tubercle.


7b. Ambulacral plates with primary tubercle and smaller secondary tubercle immediately above and perradial to it.

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8a. Plates densely tuberculate. Ornament confined to pits around primary tubercles. Primary tubercles non-crenulate.


8b. Plates sparsely tuberculate aborally; ornament consisting of sparse radial bars around primary tubercles. Primary tubercles weakly crenulate.

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9a. Plate ornament of reticulate epistroma highly developed.


9b. Plate ornament restricted to sparse radial bars around primary tubercles.