Pawson, David L Department of Invertebrate Zoology, Smithsonian Institution, Washington, DC.
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An order of regular sea urchins (class Echinoidea) that have a stirodont Aristotle's lantern (jaw apparatus) with the epiphyses not meeting at the top of the lantern, and with keeled teeth that are T-shaped in cross section. The ambulacral plates of Arbacioida are distinctive (Fig. 1); in most genera, they consist of three elements, with the central element being the largest, and the upper and lower elements being much smaller half-plates or demiplates. In some genera, there may be three or even four demiplates. The primary tubercles are large, noncrenulate, and imperforate. There are three families of arbacioids. Two of them are extinct, whereas the third—Arbaciidae—ranges from the Cretaceous to the Recent. The family Glypticidae, comprising six genera, is confined to the Jurassic era of North Africa and Europe; its ambulacral plates are compounded in a slightly different manner, with one demiplate and two larger elements. The family Acropeltidae has compound plates similar to those in Glypticidae, but it also possesses a ring of spine-carrying tubercles on the plates of the apical system. There are four genera in Acropeltidae, ranging from the Jurassic to the Miocene in Europe, the Americas, Africa, and the Middle East. The Arbaciidae is by far the largest family, with approximately ten genera, of which eight are extant. Extinct genera of Arbaciidae are essentially worldwide in distribution. See also: Echinodermata; Echinoidea
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