The most primitive of all known representatives of the earliest fishes are probably the remarkable Pleuracanthida, the genera Pleuracanthus, Xenacanthus, Orthocanthus, etc. We find well-preserved remains of them in the Permian period. Among the extinct earlier sharks of the Tertiary period there were some twice as large as the biggest living fishes; Carcharodon was more than 100 feet long.