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PROBOSCIDEA (animals " with a probosc...

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Originally appearing in Volume V22, Page 406 of the 1911 Encyclopedia Britannica.
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PROBOSCIDEA (animals " with a proboscis "), the scientific name of the group of mammals represented at the present day only by the two species of elephant. Although here regarded as a sub-order of UNGULATA (q.v.), the group is sometimes accorded the rank of an order by itself.' The existing elephants are widely sundered from all other living mammals, and for a long time palaeontology afforded but little clue as to their ancestry. Discoveries made during the first few years of the loth century in the Lower Tertiary deposits of the Fayum district of Egypt have, however, brought to light the existence of several kinds of primitive proboscideans which serve to link the group with other ungulates, and likewise apparently indicate affinity with the Sirenia. The following are some of the leading characteristics of existing elephants. The combined upper lips and nose are produced into a long muscular, flexible and prehensile proboscis, or trunk, with the nostrils at its tip. The teeth consist of a pair of large upper permanently growing incisors or tusks; and a set of cheek-teeth having their crowns composed of a series of tall transverse vertical plates gradually increasing in number from the first to the last of the series; and only portions of two of these teeth being in use at any one time. There are no clavicles; and the limbs are stout, with their component segments placed nearly in a vertical line, and the upper segment, especially in the hind-limb, the longest; the radius and ulna are distinct, the latter articulating extensively with the carpus; the fibula and tibia also distinct; the astragalus very flat on both surfaces; and both front and hind feet short, broad and massive, with five toes (though the outer pair may be more or less rudimentary), all encased in a common integument, though with distinct, broad, short hoofs; third digit the largest. Two anterior venae cavae entering the right auricle. Stomach simple. A capacious caecum. Testes permanently abdominal. Uterus bicornuate. Placenta deciduate and zonary. Teats two, pectoral. In order to understand the peculiar nature of the dentition, it is necessary to discuss to some extent those of the immediate ancestors of the true elephants, such as the mastodons (see Dentition. MASTODON). As regards the incisors, or tusks, which ' Cuvier's order Pachydermata (Gr. 7raxhs, thick and Sepµa, skin), containing the elephants, hippopotami, rhinoceros, swine, tapirs, hyraxes, &c., is now abandoned, its members now forming the orders Proboscidea and Hyracoidea and the sub-order Parissodactyla. A few Artiodactyla are also included. and generally curved, these are composed mainly of solid dentine, the fine elastic quality and large mass of which renders it invaluable as " ivory " for commerce and the arts. A peculiarity of the dentine of the Proboscidea is that it shows, in transverse fractures or sections, fine lines proceeding in the arc of a circle from the centre 1 ZI' IV, Elephas primigenius. to the circumference in opposite directions, and forming by their decussations curvilinear lozenges, as in the " engine-turning " of the case of a watch. The enamel-covering in existing species is confined to the extreme apex, and very soon wears off, but in some extinct species it forms persistent longitudinal bands of limited breadth. The tusks have small milk-predecessors, shed at an early age. As regards the cheek-teeth, these are composed in the mastodons of a variable number of enamel-covered transverse ridges, often divided into inner and outer columns, which may partially alternate, and complicated by smaller additional columns; but in the unworn tooth they stand out freely on the surface of the crown, with deep valleys between (fig. r, I). In the elephants the ridges are increased in number, and consequently become narrower from before back-wards, while they are greatly extended in vertical height. In order to give solidity to what would otherwise be a comb-like tooth, the whole structure is enveloped and united in a large mass of cement, which completely fills the valleys, and gives a general smooth appearance to the unworn tooth; but as the wear consequent upon the masticating process proceeds, the alternate layers of tissue of different hardness—cement, dentine and enamel—which are disclosed upon the surface form a fine and efficient grinding instrument. The intermediate stages between the molar of a modern elephant and that of a mastodon are so fully known that it is not possible to draw a definite line between the two types of tooth-structure (see fig. I, II, III, IV). As regards the mode of succession, that of modern elephants is very peculiar. During the complete lifetime of the animal there are but six cheek-teeth, which it will be convenient to allude to as molars, on each side of each jaw, with occasionally a rudimentary one in front, completing the typical number of seven. The last three represent the molars of ordinary. mammals, those in front are milk-molars, which are never replaced by permanent successors, the whole series gradually moving forwards in the jaw, and the teeth becoming worn away and their remnants cast out in front, while development of others proceeds behind. The individual- teeth are so large, and the processes of growth and destruction by wear take place so slowly, that not more than one, or portions of two, teeth are ever in place and in use on each side of each jaw at otn time, and the whole series of changes coincides with the usual duration of the animal's life. On the other hand, the earlier representations of the proboscidean series referred to below have the whole of the cheek-teeth in place and use at one time, and the milk-molars vertically displaced by premolars in the ordinary fashion. Among mastodons transitional forms occur in the mode of succession as well as in structure, many species showing a vertical displacement of one or more of the milk-molars, and the same has been observed I, Mastodon americanus; II, Elephas (Stegodon) insignis; to N PROBOSCIDEA sole. The hind foot is smaller and narrower than the front. The liver is small and simple, and there is no gall-bladder. In form the brain resembles that of the lower orders of mammals in that the cerebellum is entirely behind and uncovered by the cerebrum, but the hemispheres of the latter are richly convoluted. Elephants are exclusively vegetable-feeders, living, chiefly on leaves and young branches of forest trees and various kinds of herbage, or roots, which they gather and convey to their mouth by a very mobile proboscis, an organ which combines in a marvellous manner strength with dexterity of application, and is a necessary compensation for the shortness and inflexibility of the neck, as it is by this that many of the functions of the lips of other animals are performed. By its means elephants are enabled to drink without bending the head or limbs, The end of the trunk being dipped, for instance, into a stream or pool, a forcible inspiration fills the two capacious air-passages in its interior with water, which, on the tip of the trunk being turned upwards and inserted into the mouth, is ejected by a blowing action, and swallowed. Or if the animal wishes to refresh and cool its skin, it can throw the water in a copious stream over any part of its surface. Elephants can also throw dust and sand over their bodies by the same means and for the same purpose, and they have frequently been observed fanning themselves with boughs held in the trunk. The following are the distinctive features of the genus Elephas, the type of the family Elephantidae: Dentition: i. o, c. o, m. 1=26. The incisors variable, but usually of very large size, especially in the male sex, directed somewhat outwards, and curved upwards, without enamel except on the apex before it is worn; preceded by small milk-incisors. The molars succeed each other by horizontal replacement from before backwards, never more than one or part of two being in use on each side of each jaw at the same time; each composed of numerous flattened enamel-covered plates or ridges of dentine, projecting from a common many-rooted base, surrounded and united together by cement. The number of plates increases from the anterior to the posterior molar in regular succession, varying in the different species, but the third and fourth (or the last milk-molar and the first true molar), and these only, have the same number of ridges, which always exceeds five. Skull of adult very high and globular. Lower jaw ending in front in a deflected, spout-like symphysis. Vertebrae: C. 7, D. 19-21, L. 3-4, S. 4, C, 26-33. The two existing species of elephant are the Indian or Asiatic (Elephas maximus), and the African (E. africanus), the distinctive characteristics of which are given under ELEPHANT. See also MAMMOTH and MASTODON.
End of Article: PROBOSCIDEA (animals " with a proboscis ")
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