PLESIOSAURUS. The discovery of this genus forms one of the most important additions that geology has made to comparative anatomy. Baron Cuvier deemed the structure of the Plesiosaur "to have been the most singular, and its cha racters the most anomalous that had been discovered amid the ruins of a former world." "To the head of a lizard it united the teeth of a crocodile, a neck of enormous length, resembling the body of a serpent, a trunk and tail having the proportions of an ordinary quadruped, the ribs of a chameleon, and the paddles of a whale" (fig. 71). " Such," writes Dr. Buckland, "are the strange combinations of form and structure in the Plesiosaurus, a genus, the remains of which, after interment for thousands of years amidst the wreck of millions of extinct inhabitants of the ancient earth, are at length recalled to light by the re searches of the geologist, and submitted to our examination, in nearly as perfect a state as the bones of species that are now existing upon the earth." The first remains of this animal were discovered in the lies of Lyme Regis about the year 1822, and formed the subject of the paper by the Rev. Mr. Conybeare (afterwards dean of Llandaif), and Mr. (afterwards Sir Henry) De la Beche, in which the genus was established, and named Plesiosaurus (" approximate to the Saurians "), from the Greek words plesios and sauros, signifying " near " or " allied to," and " lizard,' because the authors saw that it was more nearly allied to the lizard than was the Ichthyosaurus from the same formation.
The entire and undisturbed skeletons of several individuals, of different species, have since been discovered, fully confirm ing the sagacious restorations by the original discoverers of the Plesiosaurus.
Vertebral Column.—The vertebral bodies have their ter minal articular surfaces either flat or slightly concave, or with the middle of such cavity a little convex. In general the bodies present two pits and holes at their under part. The cervical vertebrae consist of centrum, neural arch, and pleur apophyses. The latter are wanting in the first vertebra ; but both this and the second have the hypapophyses. The cervical ribs are short, and expand at their free end, so as to have suggested the term "hatchet-bones" to their first discoverers. They articulate by a simple head to a shallow pit, which is rarely supported on a process, from the side of the centrum ; but is commonly bisected by a longitudinal groove, a rudi mental indication of the upper and lower processes which sustain the cervical ribs in Crocodilia.
The body of the atlas articulates with a large hypapophysis below, with the neurapophysis above, with the body of the axis behind, and with part of the occipital condyle in front ; all the articulations save the last become, in Plesiosaurus pachyomus, and probably with age in other species, obliterated by anchylosis. The hypapophysis forms the lower two-thirds,
the neurapophysis contributes the upper and lateral parts, and the centrum forms the middle or bottom of the cup for the occipital condyle. The second hypapophysis is lodged in the inferior interspace between the bodies of the atlas and axis ; it becomes anchylosed to these and to the first hypapophysis. The first pleurapophysis, or rudimental rib, is developed from the centrum of the axis.
As the cervical vertebrae approach the dorsal, the lower part of the costal pit becomes smaller, the upper part larger, until it forms the whole surface, gradually rising from the centrum to the neurapophysis (fig. 71).
The dorsal region is arbitrarily commenced by this vertebra, in which the costal surface begins to be supported on a di apophysis, which progressively increases in length in the second and third dorsal, continues as a transverse process to near the end of the trunk ; and on the vertebra above or between the iliac bones, it subsides to the level of the neurapophysis. In the caudal vertebra the costal surface gradually descends from the neurapophysis upon the side of the centrum ; it is never divided by the longitudinal groove which, in most Plesiosauri, indents that surface in the cervical vertebra. The neural arches remain long uuanchylosed with the centrum in all Plesiosauri, and appear to be always distinct in some species. The pleurapophyses gain in length, and lose in terminal breadth, in the hinder cervicals ; and become long and slender ribs in the dorsal region, curving outwards and downwards so as to encompass the upper two-thirds of the thoracic abdominal cavity. They decrease in length and curvature as they approach the tail, where they are reduced to short straight pieces, as in the neck, but are not terminally expanded ; they cease to be developed near the end of the tail. The luemapophyses in the abdominal region are subdivided, and with the hrnal spine or median piece, form a kind of " plastron " of trans versely-extended, slightly bent, median and lateral, overlapping, bony bars, occupying the subabdominal space between the coracoids and pubicals. In the tail the hamapophyses are short and straight, and remain re-united both with the centrum above and with each other below. The • hxmal spine is not developed in this region. This modification has been expressed by the statement that there were no chevron-bones in the Plesiosaur. The tail is much shorter in the Plesio- than in the Ichthyo-saurus.