March 2, 1876] 



NA TURE 



351 



Artiodactyle nor Perissodactyle type of that order. It 

 has been thought that they possess some, though cer- 

 tainly not very close, affinities with the Rodentia, and 

 also with the Sirenia. It is certain, however, that the 

 two species of Elephant which are the sole living repre- 

 sentatives of the order, stand quite alone among existing 

 mammals, widely differing from all others in many parts 

 of their structure, being in some respects, as in the skull, 

 dentition and proboscis, highly specialised, though in 

 others, as in the presence of two anterior ven^ cavae and 

 in the structure of the limbs, retaining a low or gene- 

 ralised condition. A considerable series of extinct forms, 

 extending back through the Pliocene and Miocene epochs, 

 show the same type under still more generalised outlines. 

 Though no true Proboscidians have as yet been found in 

 any Eocene formations, certain recently discovered forms 

 of that epoch from North America, if their affinities are 

 rightly interpreted, may link them to some unknown 

 primitive type of Perissodactyle Ungulate. The consider- 

 ation of these will, however, be reserved until the next 

 lecture. 



All the true Proboscidea are arranged in three principal 

 groups or genera — Elephas, Mastodon, and Dinotherium, 

 Their molar teeth, by which the extinct species are chiefly 

 known, present a remarkable series of modifications, from 

 the comparatively simple tapiroid teeth of Dinotherium, 

 with two or three strongly pronounced transverse ridges, 

 and a normal mode of succession, to the extremely com- 

 plex structure and anomalous mode of replacement found 

 in the true Elephants. The intermediate conditions occur 

 in the various species of Mastodon. In this genus the 

 enamel-covered transverse ridges of each tooth are gene- 

 rally more numerous than in Dinotherium, and often 

 complicated by notches, or by accessory columns attached 

 to them, but in the unworn tooth they stand out freely 

 from the crown, with deep valleys between. In the Ele- 

 phants the ridges are still further increased in number, 

 and are greatly extended in vertical height, so that, in 

 order to give solidity to what would otherwise be 

 a laminated or pectinated tooth, it becomes necessary 

 to envelop and unite the whole with a large mass of 

 cementum, which completely fills up the valleys, and 

 gives a general smooth appearance to the organ when 

 unworn ; but as the wear consequent upon the masticating 

 process proceeds, the alternate layers of tissue of different 

 hardness, cement, dentine, and enamel, which are dis- 

 closed on the surface, form a fine and very efficient 

 triturating instrument. The modification of the tooth of 

 a Mastodon into that of an elephant is therefore precisely 

 the same in kind as that of the molar of a Palaeotherium 

 into that of a horse, or of the corresponding tooth of an 

 early Artiodactyle into that of an ox. The intermediate 

 stages, moreover, even as our knowledge already extends, 

 are so numerous that it is not possible to draw a definite 

 line between the two types of tooth structure. As regards 

 the mode of succession, that of the modern elephants is, 

 as before mentioned, very peculiar. During the complete 

 lifetime of the animal there are but six molar teeth on 

 each side of each jaw, with occasionally a rudimentary 

 one in front, completing the typical number of seven. 

 The last three represent the true molars of the ordinary 

 mammals, those in front appear to be milk molars, which 

 are never replaced by permanent successors, but the 

 whole series gradually moves forwards in the jaw, and 

 the teeth become 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 take place so slowly, that not 

 more than one, or portions of two teeth, are ever in place 

 and in use in each side of each jaw at one time. On the 

 other hand the Dinotherium, the opposite extreme of the 

 Proboscidian series, has the whole of the molar teeth in 

 place and use at one time, the milk molars having been 

 vertically displaced by pre-molars in the ordinary fashion. 



Among Mastodons transitional forms occur in the mode 

 of succession as well as in structure, many species show- 

 ing a partial vertical displacement of the milk molars, 

 and the same has been observed in one extinct species of 

 Elephant {E. plani/rons). 



Abundant remains of fossil elephants have been found 

 in Pleistocene and Pliocene deposits in many parts of 

 Europe, including the British Isles, in North Africa, 

 throughout the North American continent from Alaska 

 to Mexico, and extensively distributed in Asia. These 

 species are chiefly known and characterised at present by 

 the teeth, some of which resemble the existing Indian, 

 and some the African type, but the majority are interme- 

 diate between the two, and make the distinction between 

 Elephas and Loxodon as different genera quite imprac- 

 ticable. Others, again, approach so closely in breadth 

 and coarseness of the ridges, and paucity of cementium 

 to the Mastodons, as to have been placed by some natu- 

 ralists in that genus. These form the group or sub-genus 

 called Stes^odon by Falconer. The best known extinct 

 species of elephant are E. primis^enius, the Mammoth, 

 very closely resembHng the existing Indian Elephant, of 

 Pleistocene age, extensively distributed throughout Nor- 

 thern and Central Europe, North Asia, and North 

 America, though most of the remains attributed to it in 

 the latter country may belong to another species, E. 

 Americanus, De Kay, which, according to Leidy, includes 

 E. Cohimbi, Falc. In the frozen soil of North Siberia, 

 complete animals, with the flesh and hair upon them, are 

 often found, and their tusks are still collected in large 

 quantities and in a sufficiently perfect condition to be 

 used as ivory. E. atitiquus and JE. meridionalis are two 

 other species found in Britain as well as Europe gene- 

 rally, of rather earlier date, and inclining more to the 

 Loxodon type, as also do two species found in the island 

 of Malta, E. mnaidriensis and E. tnelitensis, the latter 

 the smallest known species of the group, sometimes not 

 exceeding three feet in height when adult. The Stegodon 

 forms, E. clifti, bombifrons, and insignis of Falconer and 

 Cautley, are all from India, which would appear, from the 

 abundance of remains, variety of form, as well as the 

 generalised characters of some, and the geological horizon 

 (Lower Pliocene) to be the earliest habitation of the true 

 elephant yet discovered. 



The Mastodons are distinguished from the elephants 

 principally by the form of the molar teeth ; the only 

 absolutely distinguishing character, and that somewhat of 

 an arbitrary one, being that the third, fourth, and fifth 

 molars have an equal number of ridges, whereas in the 

 elephants the third and fourth are alike, but the fifth has 

 a larger number. In addition to the great incisor tusks 

 of the upper jaw they often, but not invariably, have lower 

 incisors, which are always wanting in the elephants. 

 They are all gigantic animals, equalling or exceeding the 

 recent elephants in size. Their remains have been found 

 in Europe and Southern Asia in Miocene and Pliocene 

 beds, but not of an earlier or later date. Two species, 

 M. arvernensis and M. borsoni, occur in England in the 

 Suffolk Red Crag. They have also been found in both 

 South and North America. In the latter, M. Arnericatms, 

 Cuv., M. ohioticus, Blumenb., with very simple ridged 

 molars, is of Pleistocene age, its remains being found in 

 great abundance and very perfect condition. It was, 

 therefore, the last survivor of the genus. The Mastodons 

 were divided by Falconer into three series called respec- 

 tively Trilophodon, Tetralophodon, and Pentalophodon, 

 according to the number of ridges upon the molar teeth. 



The Dinotheria were also animals of elephantine 

 proportions, strikingly characterised by the pair of huge 

 tusks descending nearly vertically from the fore part of 

 the lower jaw. The presence or absence of upper incisors 

 has not yet been satisfactorily ascertained. The cranium 

 was much depressed, differing from that of the elephants 

 in the comparatively little development of the air-cells. 



