PROBOSCIDEA, the scientific name of an order of Euthe rian mammals represented at the present day by the African and Indian elephants. These animals (see ELEPHANT) differ very widely in their structure from all other living mammals but fossil remains of similar structure have long been known. Only during the past 20 years, however, have primitive Proboscidea been dis covered capable of comparison with other mammalian forms. The leading characteristics of the living elephants are as follows.
The animal is of large size with pillar-like limbs. The neck is short and the large head is provided with a proboscis, a long flex ible muscular organ capable of being turned freely in all directions, and provided with one or two finger-like processes at its tip which can be used to handle articles as small as a penny. The trunk can also be employed as a prehensile organ by being wrapped as a whole round a large object. It is capable of very considerable accuracy of movement and is used in feeding. The trunk repre sents the whole anterior part of the face, that is the nose and also the upper lips, the strip of skin along its ventral surface being essentially a part of the palate. The two nostrils lie at the extrem ity of the trunk and lead into great canals which perforate the whole of that organ opening into olfactory chambers at its base. The mouth is short, placed below the trunk and provided with thick fleshy lower lips meeting anteriorly so as to form a short spout. The eye is small, laterally directed and provided with a pair of eyelids often bearing stiff eyelashes. The pinna of the external ear is large, becoming enormous in certain races of the African elephants. It is usually carried close to the side of the neck but can be erected so as to stand out perhaps a yard from the side of the head. The female bears two teats placed on the pectoral surface just between and behind the forelegs. Most living adult elephants possess very few hairs; there are normally a certain num ber on the forehead and cheek and always a tuft of long extremely thick hairs at the end of the short tail.
The skeleton of an elephant is as peculiar and characteristic as his external appearance. The skull is very short from back to front, deep, and built up of spongy bone full of air-spaces. Within the great mass of bone there lies a relatively small, though actually large brain cavity, and the olfactory chambers form great perfora tions which with the naso-pharyngeal ducts penetrate obliquely through the skull uniting only quite posteriorly. The skull of the new-born elephant differs from that of the adult in that whilst the head increases greatly and disproportionately in size, the brain undergoes much less expansion. Thus during the period of growth
of the animal the external surface of the skull, by the development of the great air-spaces in its middle layer, becomes more and more widely separated from the layer of hard bone which surrounds the brain. The bony lamellae which separate these air-spaces run radially so as to buttress the external surface. The great size of the adult skull is necessary in order to give adequate areas for the attachment of the immense neck-muscles which support and move the very heavy trunk and tusks, and take the great strains which are produced when these structures are used in digging up and tearing down trees.
The jaw-bones of the elephant and the whole structure of its palate are modified so as to receive and afford adequate support to the tusks and cheek teeth. The bony nostrils lie very high up and are overhung by very small nasal bones to which some of the muscles of the base of the trunk are attached. This attachment is brought as far back as possible in order to increase the range of action of the proboscis.
The dentition of an adult elephant consists of a single pair of exaggerated incisors, the tusks, and either one or two molars on each side of the upper and lower jaws. These molars are built up of a series of plates each composed of a core of dentine surrounded by a layer of enamel; the individual plates, continuous with one another at their base, are held together and supported by an infill ing of cement. In the elephant the milk incisors, little teeth about two inches in length, are shed and replaced by the permanent tusks, but the milk molars instead of being pushed out by a permanent pre-molar which develops underneath them, replace one another from behind as follows. The new tooth is developed deep down in the posterior part of the jaw and travels obliquely forwards and towards the mouth, so that more and more of it cuts the gum as the preceding tooth is worn down to its root, until finally the latter is shed and its successor is well in wear. A living elephant in this way works through six teeth in each side of each jaw during its life time, wearing down a total thickness of nearly a yard of tooth over an area which increases from about half a square inch to nearly 3o sq. in. on each side of its mouth. This extraordinary dentition is unparalleled, but the process by which it came into existence is fortunately completely known from fossil material.