Though the plesiosaurs are nearly perfectly adapted to an aquatic life, the adaptation was, in many respects, of a very different kind from that of the ichthyosaurs—so very different that we have not yet quite finished conjecturing as to the habits of the living animals. As already suggested in the popular name, the most striking characteristic of the typical plesiosaurs, the one which suggested to Buckland his frequently quoted simile, is the ofttimes enormously long neck, proportionately longer than that of any other known creatures of the past or present. In other truly aquatic animals the neck is actually shortened in the acquirement of a fish-like shape, and the number of bones composing it reduced. In the Sauropterygia the neck is usually longer than any truly land animals ever possessed, the longest-necked forms having as many as seventy-six vertebrae in the cervical region. The elongation of the neck among mammals is always due to an increase in the length of the individual bones, never to an increase in the number from seven, with but a single exception—a South American sloth which has nine cervical vertebrae. The long neck of birds is due both to an increase in the length of the individual vertebrae and to an increase in their number, to as many as twenty-one. But the elongation of the neck among plesiosaurs was very variable indeed; sometimes it was ten or twelve times the length of the head, at other times it was even shorter than the head. And the number of bones composing it was also extremely variable, scarcely any two species having the same, the known extremes being seventy-six and thirteen. In _Elasmosaurus platyurus_, for instance, the longest-necked plesiosaur known, the head was two feet in length, the neck twenty-three, the body nine, and the tail about seven; on the other hand, in the shortest-necked plesiosaur known, _Brachauchenius Lucasi_, the head was two and one-half feet in length, the neck less than two feet, and the body about five; the length of the tail is unknown. [Illustration: FIG. 32.—Skeleton of _Trinacromerum osborni_, a Cretaceous plesiosaur, as mounted in the University of Kansas Museum.] Not only was the number of vertebrae so extraordinarily increased in many plesiosaurs, but in the longest necks the vertebrae themselves, as in birds, were more or less elongated, especially the posterior ones, which may be six or seven times the length of the anterior ones. Not only was the neck of such great length in many plesiosaurs, but it also tapered very much toward the head. The vertebrae are always biconcave, but the cavities are shallow, saucer-like, sometimes almost flat at each end, and very different from the conical fish-like cavities of ichthyosaurian vertebrae. [Illustration: FIG. 33.—Restoration of _Elasmosaurus platyurus_, an Upper Cretaceous plesiosaur.]