Arbacioida
Arbacioida (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.
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Arbacioida