The Echinoid Directory

Key to major groups of Meridosternata

1a. Plastron meridosternous: episternal plates offset.
Basal Meridosternata

1b. Plastron orthosternous: episternal plates more or less paired.

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2a. Fascioles absent in adults.

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2b. Fasciole present.

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3a. Subanal fasciole present.

Urechinina

3b. Marginal fasciole present.

Stegasterina

4a. Apical disc disjunct and appears compact with posterior genital pores on ocular plates II and IV.

Pourtalesiidae (Urechinina)

4b. Apical disc standard in structure, with posterior genital plates bearing gonopores.

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5a. Sharply defined frontal groove on oral surface leading from peristome to ambitus. Peristome small and facing anteriorly.

Stegasteridae (Stegasterina)

5b. Anterior convex with only slight depression in front of peristome at most.

Urechinidae (Urechinina)