PLESIOSAURUS, a genus of extinct rep tiles of the order Sauropterygia. Several species have been described from very complete re mains in European Lias rocks, first discovered in 1822 and studied by Cuvier and his followers.
In these reptiles the head is small, the neck very long, the tail of moderate length. The typical species is P. dolichodirus, about 10 feet long; allied species are sometimes found 25 feet long. There is a fine skeleton preserved in the Berlin Museum, classified as P. guileimi impera toris. Over 25 species have been recognized in the Liassic seas of England, and they are also common in the continent of Europe. In the United States related species have been located in the Upper Jurassic and Upper Cretaceous. One of these uncovered at Fort Wallace, Kans., and known as Cope's Elasmosaurus had a neck 22 feet long and a total length estimated from the incomplete skeleton of at least 42 feet. These monstrous lizards swallowed stones like modern fowl, to aid their digestion. Their food was undoubtedly fish, as their dentition was adapted to grasping and tearing. It may
reasonably be inferred that these reptiles were aquatic and inhabited the estuaries of rivers and shallow waters, occasionally swimming out to sea. Their gait on land would necessarily be awkward and difficult. The elongated neck would serve to enable them to descry their prey from among or over the tops of the reeds and vegetation amid which they might lie concealed in the banks of rivers or in estuaries, while this same length of neck would adapt them for readily seizing their prey, which consisted prob ably in greater part of fishes. Some have thought that a few remain, and that their long necks and small heads have caused them to be taken for swimming serpents, thus accounting for the ever-recurring accounts of sea-serpents. Consult for particulars of structure Woodward, 'Vertebrate Paleontology' (1898) ; and Willis ton, S. W., 'North American Pleiosaurs' (in 'Field Columbian Museum Publication 73,' Chi cago 1903).