Proboscidea

The

elephant's backbone possesses a neck of seven very short vertebrae, the number of dorsals varies from 19 to 21, the an terior having exceedingly long neural spines for the attachment of the nuchal ligament which passes forwards to the back of the skull. There is a short lumbar region of three or four vertebrae and the sacrum is a compact bone built up by the fusion of four vertebrae : there are about 3o caudals. The ribs are of enormous length sur rounding the very capacious thorax. The shoulder girdle consists of a very large triangular scapula placed vertically on the side of the thorax. The humerus is longer than the forearm so that the elbow of an elephant lies at a point relatively only a little higher than the wrist of a horse. The radius and ulna are peculiar in that their surfaces of articulation with the carpus are nearly equal, whilst the upper end of the radius is relatively small and lies in front of the ulna, the two bones crossing one another as they are traced downwards. The carpus of an elephant is unusual in that the bones of the two rows of which it is composed do not alternate but are superimposed on one another. In this feature the elephants resemble the Hyraxes. The elephant walks on the extreme tips of his fingers and toes, but the palm of the hand and the sole of the foot are swollen out into great pads of connective tissue which transmit the weight of the animal to the ground directly and so reduce the load carried by the phalanges. The elephant's pelvis is remarkable for its extraordinary width, the ilia being expanded into transversely placed sheets of bone from whose posterior sur face muscles pass down to the hind-leg, whilst their margins give attachment to the muscles of the body-wall which support the weight of the viscera within the abdominal cavity. As in the fore limb the upper segment of the leg is considerably longer than the lower one, and in the tarsus the astragalus is flattened, as a weight carrying adaptation, whilst the calcaneum is produced into a short heel directed backwards and downwards.

The anatomy of the soft parts of the elephant has been repeat edly described but presents few features of special interest except those which result from the modification of the nose and upper lips into the trunk. The animal is peculiar in that there is no pleural cavity. The fact that the testes are abdominal, the uterus bicor nuate, and the placenta zonary and deciduate are of importance because similar conditions occur in the Sirenia which may have sprung from an early Proboscidean stock.

The most primitive and earliest known ancestors of the elephant belong to the genus Moeritherium and are found in Upper Eocene deposits in the Fayilm of Egypt. Moeritherium was an animal resembling in its external appearance and size the living tapir. The eye was placed far forward and the head was low and elon gated, probably the end of the snout was slightly flexible and it may have been produced into a short proboscis. The legs were con siderably more bent than those of the modern elephants hut are still rather incompletely known. Morritherium possesses three up per and two lower incisors of which the second pair are enlarged, those in the upper jaw projecting straight downwards, whilst the lower teeth are directed forwards so that their tips bite against those of the upper incisors. There is a small and very reduced canine tooth; three pre-molars are present in each jaw and there is evidence that each of these vertically replaced a milk predeces sor. Three permanent molars occur in both upper and lower jaws, each consisting of two transverse ridges.

The next stage, represented by Palaeomastodon from the Lower Oligocene of Egypt, presents a great advance on Moeritherium. There are several species of which the largest is not very much smaller than a small elephant, whilst the smallest is little bigger than the largest Moeritherium. The back of the skull begins to show the separation of the outer surface from the brain-cavity by air-spaces, which is carried to an extreme in the living elephants. The bony nostril has shifted backwards to a point in the middle of the cheek-teeth, and in front of it the pre-maxillae form an open channel in which lay the base of the trunk. The second incisor is very much enlarged and directed forwards and down wards. The lower jaw is so much longer than the skull that the incisors, which lie horizontally, project forwards several inches in front of the upper jaw, and even extend beyond the tips of the upper incisors. The single "second" lower incisor is enlarged, and with its fellow forms a shovel-shaped termination to the lower jaw. The shape of this tooth is such as to show that its upper surface was worn by contact with some part of the animal and the only explanation of its structure is that the nose, upper lip and palate projected so far forward as to overhang the front of the ' lower jaw and form there a movable proboscis. The lower incisors are, however, worn all round in a way which suggests that the creature used them for grubbing about in the ground to secure food. There are three upper pre-molars each of which replace a milk tooth, whilst in the lower jaw the three milk molars are re placed only by two pre-molars. Both upper and lower molar teeth have three transverse ridges, thus differing from those of Moerith erium. The body of the animal was much like that of a small elephant but the neck was longer.

In the next stage, which is represented by Tetrabelodon angus tidens from the Lower and Middle Miocene of Europe, North Africa and Baluchistan, we have an animal somewhat larger than the largest Palaeomastodon, with a completely elephant-like body, though with a somewhat longer and more flexible neck. The skull is much more elephant-like than that of Palaeomastodon because of the increased bulk of the air-cavities in its bones. It supports a pair of immense incisors, which, unlike the tusks of living ele phants, are still down-turned and are provided with a belt of enamel lying on the outer surface of the ivory of which they are composed. The lower jaw is even longer than in Palaeomastodon nearly half its length projecting in front of the bony skull. The lower tusks are directed forwards and bear a wear facet on their upper surfaces made by friction against a pad on the flexible and unsupported anterior part of the face, soon to be, if it had not al ready become, a true trunk. Tetrabelodon still possesses milk molars which are replaced vertically by pre-molars, but these latter are comparatively small teeth soon displaced by the cutting of the second and third molars, which push their way forward from the cavity in the hinder part of the jaw in which they are formed. This process is carried so far that the adult has only two molars, the second and third, in position in each jaw. The individual teeth though larger than those of Palaeomastodon have still only three ridges, except in the case of the third, which has five. It seems evident that the enormously elongated jaws of Tetrabelodon devel oped as an adaptation to allow the animal to reach the ground, when as a whole it was increasing in height whilst its neck was becoming shorter.