EVOLUTION OF THE PKOBOSCIDEA. 



10!) 



the differences are not great, the same mechanical conditions having existed in both 

 forms. In Tetrabelodon the upper tusks are somewhat less unlike ordinary incisors 

 than in Elephas, in being curved downwards and in bearing a continuous band of 

 enamel on their outer face. As in Elephas they are separated by a considerable 

 interval at their alveolar ends and diverge widely distally. 



It is in the mandible that the greatest differences between the two genera are 

 found. In Elephas the symphysial region has undergone reduction to such an extent 

 that it forms merely a very short downwardly directed spout -like projection. In 

 Tetrabelodon, on the other hand, the most remarkable condition of things prevails (see 

 figs. 8 and 9). The enormously elongated symphysial region bearing a pair of 

 procumbent incisors, projects far beyond the premaxillse and between the divergent 

 upper tusks (see figs. 6 and 7), with which the lower incisors cannot have been in contact 

 at any time ; nevertheless, these lower teeth bear on their extremities flat surfaces ot 

 wear, both on their upper and lower sides, a condition that can only be accounted for 

 by supposing that they were employed for digging and rooting in the earth. The great 

 length of the mandible, combined possibly with a somewhat longer neck than is found 

 in Elephas, rendered it possible for this animal to reach the ground with its lower 

 incisors. No doubt the portion of the mandible projecting beyond the upper jaw was 

 covered by a sort of proboscis formed by the combined nose and upper lip (compare 

 figs. 7 with 9 and 6 with 8) ; the extremity of this must have been more or less prehen- 

 sile and capable of thrusting the food, rendered available by the action of the incisors, 

 into the channel formed by the symphysial region of the mandible. Subsequently, 

 it must be supposed, that owing to some change in the nature of the food and probably 

 also to the increased efficiency of the end of the proboscis as an organ of prehension, 

 the mandible became shortened so that it was no longer possible for the animal to 

 reach the ground with its lower incisors, and when this happened the elongated 

 symphysis would not only cease to be of any service, but possibly even prove a 

 disadvantage as interfering with the free action of the proboscis, and therefore it was 

 rapidly lost. This may account for the fact that some species of Mastodon (e.g., 

 M. americanus, see fig. 5), but little advanced beyond Tetrabelodon angustidens in 

 the structure of their molars, have the symphysis nearly as short as in Elephas, the 

 reason apparently being that while in the case of the molars the increase of efficiency 

 was gradual, in the case of the shortening of the symphysis and the projecting 

 incisors there was a sudden transition from utility to uselessness or worse. 

 Tetrabelodon seems to represent the stage at which the length of the mandible 

 culminated, for it is an animal of much the size and build of the elephant and yet 

 could reach the ground with its lower incisors. The above explanation of the possible 

 mode of origin of the trunk in the Proboscidea was in part, so far as the main points 

 are concerned, suggested to me by Professor E. P. Lankesteii. 



