An order of Paleozoic sharklike fishes abundant in fresh-water deposits of the Carboniferous and Early Permian. Although primitive in cranial structures, the pleuracanths differ notably from other sharks in several regards (see illustration). The teeth are two-pronged; there is a long spine projecting upward and backward from the posterior part of the braincase; the tail extends directly backward, in contrast with the upturned heterocercal caudal fin of other sharks; and the paired fins have the archipterygial skeletal pattern of a central axis and side branches, in contrast with a fan-shaped arrangement in typical sharks. See also Elasmobranchii.

Xenacanthus (Pleuracanthus), Carboniferous and Permian sharklike form; perhaps 2½ft (75 cm) long. (After Fritsch)

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