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(class Echinoidea, superorder Echinacea)

An order of regular echinoids in which the imperforate, non-crenulate primary tubercles are poorly developed. Adventitious skeletal material on the test surface, simulating tubercles, is commonly developed, but does not bear spines. Arbacioids first appeared in the Middle Jurassic.

Subjects: Zoology and Animal Sciences.

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