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The Echinoid Directory

Key to stem group Salenioida genera

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1a. Apical disc monocyclic.
 1b. Apical disc hemicyclic.

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2a. Ambulacral plating trigeminate throughout (one primary tubercle to every three pore-pairs).

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2b. Ambulacra with unigeminate plating adapically (one tubercle adjacent to each pore-pair).

Metacrosalenia

3a. Adapical ambulacral and interambulacral tubercles abruptly become small and rudimetary; upper surface covered in dense uniform granules.

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3b. Adapical ambulacral and interambulacral tubercles reduce in size gradually; upper surface not covered in dense uniform granules.

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4a. Apical disc with thin U-shaped genital plate 5; no phyllodes adorally; pore-pairs uiserial to peristome
Milnia

4b. Prominent phyllodes developed over oral surface (apical disc unknown)
5a. A single large suranal plate fills the centre of the disc.

 5b. A pavement of polygonal periproctal plates lie inside the apical disc.