170 



KNOWLEDGE. 



[August, 1903. 



lip in the jaws in the arc of a circle. By this arrangement 

 it results that there are never more tlian two teeth in use 

 at any one time on each side of both jaws ; while in old 

 asfe there is but a single pair of cheek-teeth in each jaw. 

 When these last become so worn down as to be useless, 

 the elephant must die ; although it is probable that, in 

 most cases, death takes palace before such a completely 

 toothless condition is reached. 



It is thus evident that modern elephants, far from being 

 primitive as regards their dentition, are liiglily specialized 

 creatures. The nearest approach to this remarkable mode 

 of dental succession is presented, among living mammals, 

 by the manati, or sea-cow, in which an almost endless 

 series of cheek-teeth come up one behind another in the 

 jaws as those in front are worn out and discarded. In 

 connection with this resemblance in the mode of succession 

 of the teeth of the manati to the condition obtaining in 

 the elephants, it is important to notice that the ancestral 

 elephants, or rather proboscideans, appear nearly related 

 to the progenitors of the manati. In the early ancestral 

 forms of both groups the cheek-teeth were of a uonnal 

 type as regards their mode of succession, the whole series 

 being in use at once. Consequently, the peculiar mode of 

 succession occurring respectively in the modern elephants 

 and manatis must have been independently acquired in 

 each. Cases of such parallelism in development are common 

 enough in groups of widely diverse origin, but the occur- 

 rence of the phenomenon in groups closely connected is a 

 very remarkable circumstance. 



As already mentioned, the component plates of the 

 cheek-teeth of the African elephant are fewer in number 



Fia. 1.— Grinding Surface of a Lower Molar of the African Elephant. 



and shorter and thicker in form than those of the Asiatic 

 elephant and mammoth. Extinct forms show, however, a 

 complete transition in this respect between the two types. 

 On the other hand, there is a gradation in dental structure 

 from the African elephant towards the extinct group of 

 stegodons, or ridge-toothed elephants, of which the fos- 

 silised remains are met with abundantly in late Tertiary 

 formations from India and Central Asia to Japan and 

 Java, but have hitherto been found in no other part of the 

 world. In these ridge-toothed elepliants, as their scientific 

 name of stegodon implies, the plates of the cheek-teeth are 

 reduced to low transverse ridges, recalling the pitch of a 

 slate roof. Each ridge is separated from its neighbour bv 

 an open V-shaped valley; and the number of ridges in 

 each tooth is much less than in the corresponding tooth of 

 the true elephants. A further peculiarity is to be found 

 in the circumstance that in the third, fourth, and fifth 

 pairs of teeth the number of ridges is nearlv the same. 

 This equality in the number of ridges in their "inter- 

 mediate molars," as they have been designated, forms a 

 connection between the ridge-toothed elephants and the 

 mastodons. 



Owing to the miUstone-like surfaces formed by their 

 cheek-teeth, modern elephants masticate their food by a 

 backwards-and-forwards motion of their jaws. Obviously, 

 however, such a movement would be incompatible with 



transverse ridges separated by open valleys on the summits 

 of the cheek-teeth ; and it is consequently evident tliat the 

 ridge-toothed elephants, as well as their predecessors the 

 mastodons, masticated their food partly by a champing 

 and partly by a sideways movement of the jaws. Sucii 

 a difference in the method of mastication is noticeable 

 between the modern and ancient representatives of several 

 groups of hoofed animals — notably in the case of the horse 

 as compared with its progenitors. 



One of the ridge-toothed elephants, as well as another 

 extinct Asiatic species more nearly allied to the African 

 elephant, displays a peculiarity in its dentition which tends 

 to show the derivation of the group from animals of a less 

 specialised type. To explain this it must be mentioned 

 that the whole six pairs of cheek-teeth developed in each 

 jaw of a modem elephant, from childhood to maturity, 

 correspond with the baby-molars plus the permanent molars 

 of the human dentition ; the only difference as regards 

 number being that the elephant has three of these baby- 

 molars, whereas the human infant possesses but two on each 

 side of both jaws. Consequently, the elephant possesses 

 no representatives of the premolars or bicuspids which 

 vertically replace the baby-molars of the human infant as 

 they become useless. That is to say, except in the case of 

 its tusks, an elephant has no vertical replacement of its 

 teeth ; in other words, it has lost the premolars common 

 to other animals. 



From the stegodons, or ridge-toothed elephants, it is 

 but a step to the mastodons, which are mainly dis- 

 tinguished by the inferior height and smaller number of 

 their transverse ridges. In the " intermediate " molars of 

 one type of mastodon, the number of ridges is four on 

 each tooth, although, as a rare abnormality, there may be 

 five. Mastodons show a step away from elephants in the 

 fact that there may sometimes be portions of three cheek- 

 teeth in use on the same side of one jaw at the same time ; 

 this being due to the smaller size of the teeth. 



Here it is important to notice that the species of 

 mastodon which presents the nearest approximation to the 

 stegodons — namely the broad-toothed mastodon — occurs 

 in the same coimtries as the latter, that is to say in 

 Northern India, Burma, &c. In this species there were 

 tusks only in the upper jaw, and the lower jaw terminated 

 in front in a short spout. As regards both these features, 



t IQ. 2. — Imperfect Skull of a four-ridged Mastodon, showing only 

 two molar teeth (the penultimate and last) in use on each side of 

 both jaws. (From a s]5ecimen in the British Museum.) 



the North American mastodon, which appears to have 

 survived into the human period, was a very similar 



