In 1821 the curator of mineralogy of the British Museum—Koenig by name—after a more critical study of other remains, reached the conclusion that these animals were intermediate between the fishes and the reptiles, and gave to them the generic name _Ichthyosaurus_, meaning fish-reptile, a name by which the chief forms have ever since been known. Within the next few years many specimens of ichthyosaurs were carefully and fully described by Conybeare, Cuvier, Owen, and others of England, France, and Germany, making very clear all the more important details of their skeletal structure. Blaineville, in 1835, thought that the ichthyosaurs constituted a distinct class of vertebrates equivalent to all other reptiles, the birds, and the mammals, which he called Ichthyosauria, the first appearance in literature of the name by which the order is properly known. Five years later, however, the famous English anatomist and paleontologist, the late Sir Richard Owen, united the ichthyosaurs with the plesiosaurs as a single order of reptiles, to which he gave the name Enaliosauria, meaning sea-reptiles, a name which has long been current in textbooks and general works on natural history. Moreover, Owen rather arbitrarily changed Blaineville’s name Ichthyosauria to Ichthyopterygia, a name which is often, though incorrectly, used to designate this order of reptiles. These briefly given and perhaps dry details will make clear how necessary is that rule of priority upon which naturalists so often insist. When anyone may change the names of organisms at will there will be no stability and no uniformity, because there is no one to decide, and the prestige of a great name, like that of Owen, will carry authority till someone else with greater authority appears. Whether or not the name _Proteosaurus_, first given to any member of this order, should take precedence over the later _Ichthyosaurus_ is still in doubt, since Home gave no specific name to his species, and the very particular purists of modern times have decided that a genus is not named unless the species is also! We moderns sometimes are inclined to impose very stringent conditions upon the older naturalists; let us hope that we shall be treated more leniently by the future naturalists!