The Echinoid Directory

Key to the families of Echinothurioida

1a. Peristomial membrane with 10 large buccal plates (arrowed) plus a few small marginal ambulacral plates.
Kamptosomatidae

1b. Peristomial membrane formed of a series of uniform, wedge-shaped ambulacral plates.

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2a. Primary spines ending in a conical hyaline hoof. Ambulacral pore-pairs arranged into three discrete columns in series.
Echinothuriidae

2b. Primary spines simple, without a terminal hyaline hoof. Ambulacral pore-pairs forming a single irregular column in each series.

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3a. Ambulacra forming broad phyllodes adorally (peristome to right in picture), with larger and smaller primary tubercles forming multicompound elements.
Pelanechinidae

3b. Ambulacra with uniserial pores adorally; primary tubercles not heterogeneous and not creating multicompound elements (peristome edge at far right).

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4a. Ambulacra with major element in each pseudocompound plate excluded from the adradial suture in ambital and adapical plates.

Paraphormosoma

4b. Ambulacra with major element in each pseudocompound plate reaching adradial suture.

Phormosomatidae