PLIOSAURUS, Ow.-M. von Meyer regards the num ber of cervical vertebrae and the length of neck as characters of prime importance in the classification of Reptilia, and founds thereon his order called Macrotrachtlen, in which he includes Simosaurus, Pistosaurus, and Nothosaurus, with Plesiosaurus. No doubt the number of vertebrae in the same skeleton bears a certain relation to ordinal groups : the Ophiclut find a common character therein ; yet it is not their essential character, for the snake-like form, dependent on multiplied vertebra, characterizes equally certain Batrachians (Celecilia) and fishes (Murcena). Certain regions of the vertebral column are the seats of great varieties in the same natural group of Reptilia. We have long-tailed and short-tailed lizards ; hut do not therefore separate those with numerous caudal vertebrae, as " Macroura," from those with few or more. The extinct Dolichosaurus of the Kentish chalk, with its proccelian vertebra, cannot be ordinarily separated, by reason of its more numerous cervical vertebra, from other shorter-necked proccelian lizards. As little can we separate the short-necked and big-headed amphicalian Pliosaur from the Macrotrachelians with which it has its most intimate and true affinities.
There is much reason, indeed, to suspect that some of the nntschelkalk Saurians, which are as closely allied to Notho saurus as Pliosaurus is to Plesiosaurus, may have presented analogous modifications in the number and proportions of the cervical vertebrae. It is hardly possible to contemplate the broad and short-snouted skull of the Simosaurus, with its proportionally large teeth, without inferring that such a head must have been supported by a shorter and more powerful neck than that which bore the long and slender head of the Nothosaurus or Pistosaurus. The like inference is more strongly impressed upon the mind by the skull of the Placodus, still shorter and broader than that of Simosaurus, and with vastly larger teeth, of a shape indicative of their adaptation to crushing molluscous or crustaceous shells.
Neither the proportions and armature of the skull of Placodus, nor the mode of obtaining the food indicated by its cranial and dental characters, permit the supposition that the head was supported by other than a comparatively short and strong neck. Yet the composition of the skull, its proportions,
cavities, and other light-giving anatomical characters, all bespeak the close essential relationship of Placodus to Simo saurus and other so-called " macrotrachelian" reptiles of the muschelkalk beds. I still, therefore, regard the fin-like modi fication of the limbs as a better ordinal character than the number of vertebra in any particular region of the spine. But by those who would retain the term Enaliosauria for the large extinct natatory group of saurian reptiles, the essential distinctness of the groups Sauropterygii and Ichthyopterygii, typified by the leldhyos«urus and Plesiosaurus respectively, should be borne in mind.
Sp. Pliosa urns brachydeirus, Ow.—The generic characters of Pliosaurus are given by the teeth and the cervical vertebra. As compared with those of Plesiosaurus, the teeth are thicker in proportion to their length, are subtrihedral in transverse section, with one side flattened, and bounded by lateral promi Pliosaurus (Kimmeridgian).
nent ridges from the more convex sides, which are rounded off into each other, and alone show the longitudinal ridges of the enamel ; these are there very well defined. The vertebra of the neck, presenting a flat articular surface of the shape shown in outline below the neck in fig. 72, are so compressed from before backward as to resemble the vertebrae of the Ichthyo saur:is (fig. 70, c), and as many as twelve may be compressed within the short neck intervening between the skull and scapular arch, as shown in fig. 72. For the rest, save in the more massive proportions of the jaws and paddle-bones, the bony framework of Pliosaurus closely accords with that of Plesiosaurus; and, as the vertebrae of the trunk resume the plesiosaurian proportions, they give little indication of the genus of reptile to which they truly belong, when found detached and apart. Some individuals of Pliosaurus appear to have attained a length of between 30 and 40 feet. The remains of this modified form of Enaliosaur are peculiar to the Oxfordian and Kinuneridgian divisions of the upper oolitic system. They have been discovered in these beds in Russia (Pliosaurus lVorinsk-ii and Spondylosaurus of Fischer), as well as in those counties of England where the Kimmeridge and Oxford clays have been deposited.