The trilobites comprise a wholly extinct group of primitive marine arthropods. They were extremely abundant in the Cambrian and Ordovician geologic periods, becoming extinct in the Permian. The flattened, oval body was composed of a head covered by a dorsal shield, a trunk (thorax), and a terminal segment (pygidium). Most of the 3,900 species ranged in length from 1 to 4 in. (2.5–10 cm); some planktonic forms were smaller, and some species were as long as 21/2 ft (76 cm). Triarthrus eatoni was a fossil trilobite common in the Ordovician seas.
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