Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/231

Rh PLESIOSAUEIANS 221 different times been found, the general proportions of the body are well known. Although the different species vary in regard to proportions, the small size of the head and extreme length of the neck are always striking points in the skeleton of a Plesiosaurus, while the tail is propor tionately short. The limbs, both fore and hind, are well developed and modified for swimming, the forms of the various bones making it clear that the digits of each limb were not separate, but enclosed in one covering of integu ment, as in the flippers of a whale or a turtle. The exterior of the body, there is every reason to believe, was smooth as it is in Cetacea, and not provided with either FIG. 1. Plesiosaurus (after Owen). rostrum. On the base of the skull four fossae are to be seen ; the front pair of these are bounded behind by the palatine bones, and are regarded as the true posterior nares. The teeth are slender, sharp, curved, and striated ; they have single fangs, and are placed loosely in separate alveolar sockets. The spinal column is composed of a large number of vertebrae, some species having ninety or more in the entire series. The centrum of each vertebra has the fore and hind surfaces slightly concave ; the neural arch is con nected with the centrum by a suture, which seems never to have been entirely obliterated. The cervical vertebrae vary in number from twenty-four to upwards of forty in different species. Each is provided with a pair of ribs, closely resembling those found in the cervical region in the crocodile, but with a single articular head only. Towards the hinder part of the neck the ribs become more elongated, and take on the form of dorsal ribs ; but, as none of the ribs join the sternum, the usual means of dis tinguishing the dorsal and cervical regions is wanting. There may be from twenty to thirty dorsal vertebras. True sternal ribs have never been detected ; but abdominal ost Fig. 2. FIG. 2.- -Pectoral arch of Plesiosaurus, seen from below (after Hulke). co, coracoid; sc, scapula; pc, precorncoid ; ost, omosternum. FIG. 3. Pelvic arch of Plesiosaurus, from above (after Huxley), is. ischium: pb, pubis ; il, ilium. bones, or ribs, are well developed. The sacrum consists of two vertebrae, with stout broad ribs for attachment to the iliac bones. The caudal vertebra?, between thirty and forty in number, have distinct chevron bones, which are attached between the successive vertebras. The pectoral arch (fig. 2) consists of a large coracoid on each side, in bony or horny scutes or scales as in the living crocodiles and turtles. The internal skeleton therefore is the only part available for study. The skull of Plesiosaurus has a tapering and depressed snout, and in consequence of the large size of the pre- maxillary bones the nasal apertures are placed far back, just in front of the orbit, as in birds. There is a distinct parietal foramen, as in lizards. The orbit is completely surrounded by bone, and there are supra- and infra- temporal fossae. The single occipital condyle is formed almost entirely by the basi-occipital bone. The basi- sphenoid is well developed, and is produced into a long front and outside of which is a peculiarly shaped scapula with a plate extending dor- sally from the glenoid cavity, and a second process directed inwards and downwards. The latter process is now regarded as the precoracoid by Mr J. W. Hulke, who also considers the plate of bone originally of two pieces found in the middle and in front of the coracoids to be the homologue of the omosternum of Batrachia. If this interpretation be correct, Plesiosaurus has neither clavicles nor interclavicles. In the fore limb all the characteristic bones are present. The humerus is an elongated bone with the anterior border nearly straight and the hinder border concave ; it is rounded at the upper end, and flat tened below, where it is articulated to two much shorter bones, the radius and ulna. Next to these is a row of three carpal bones the radiale (scaphoid), the ulnaro (cuneiform), and the intermedium (lunar) ; a second row of four bones succeeds these, three of which are carpals, but the outer one may be a metacarpal ; next comes a row of five metacarpals. The digits are five in number, and with the exception of the first are made up of numerous separate ossicles, or phalanges. The pelvic arch (fig. 3) is large, and ventrally consists of a pair of flattened more or less quadrate pubes, and a pair of somewhat triangular ischia. The iliac bones are elongated, narrower where they form part of the acetabular articula tion and becoming broader above where they join the sacral ribs. The hind limb very closely resembles the fore limb. The front and back margins of the femur are straighter than they are in the humerus ; but the other parts almost exactly repeat the corresponding bones of the fore limb. With regard to the probable habits of the Plesiosaurus we are not without some indications. The paddle-like form of the limbs leaves no doubt as to its aquatic mode of life, and judging from the fossils with which it is usually asso ciated it must have been an inhabitant of the sea ; it is highly probable, however, that some species at least ascended rivers, for remains of Plesiosauria are found in the Wealden freshwater deposits. The comparatively small tail and large paddles render it probable that the limbs were the chief means of propulsion. The long neck would tend to impede its progress through the water, and it would be better adapted, therefore, for swimming on or near the surface. It is unlikely that the Plesiosaurus could move as rapidly through the water as the Ichthyosaurus; but this slower movement would be compensated for by the rapidity with which its long and flexible neck could be darted at its prey. Seeing that the marine turtles and seals of the present day make their way on shore, it is quite possible that the Plesiosaurus may also have occasionally visited the