528 Miscellaneous. 



The mandible is at first short and stout, with a massive sym- 

 physis. Afterwards it becomes more and more elongated as the 

 stature of the animals increases ; and this elongation is for the most 

 part effected by the lengthening of the symphysial region, tbough 

 the backward rotation of the ascending ramus tends to the same 

 end. The prolongation of the mandible beyond the premaxillag 

 must have been covered by a proboscis-like structure composed of 

 the upper lip and nose, probably more or less prehensile at its extre- 

 mity. The lengthening of the mandible seems to have reached its 

 maximum degree in the [Middle Miocene, after which it again 

 became shortened by the reduction of the syrnphysis, while the 

 fleshy and now mobile proboscis was left behind as the sole organ 

 of prehension. 



In the upper dentition the chief changes are the loss of incisors 

 nos. 1 and 3 and the great increase in size of incisor no. 2, which 

 eventually forms the great tusk characteristic of the later Pro- 

 boacidea. The canines are soon lost. In the earliest forms some at 

 least of the cheek-teeth (milk-molars) are replaced by premolars in 

 the usual manner, and these teeth remain in wear simultaneously 

 with the true molars ; but in later forms no vertical succession takes 

 place, and as the milk-molars are worn they are shed, being replaced 

 from behind by the forward movement of the molars. Of these also 

 the anterior may be shed, until at length in old individuals of the 

 later types the last molar is alone functional. The gradual increase 

 in the complexity of the Proboscidean molars is one of their most 

 striking characteristics. All stages can be traced between the 

 simple, brachyodont, bilophodont (quadritubercular) molars of Moeri- 

 tlurium (Middle Eocene) to the extraordinarily complex type of tooth 

 found in Elephas. Thus in Palceomastodon (Upper Eocene) the 

 molars are trilophodout, and the same is true of the first and second 

 molars of Tetrahelodon (Miocene), in which, however, the last molar 

 is complicated by the addition of further transverse crests. In the 

 Stegodonts of the Siwalik Hills (Pliocene) a further increase in 

 the number and height of the crests takes place, and the whole 

 crown of the tooth is more or less covered with a thick coat of 

 cement. Still later the transverse crests become highly compressed 

 laminae united by cement, and these are as many as twenty-seven 

 in number in the Pleistocene Eleplias prlmir/enius and the recent 

 U. indicus. 



The evolution of the lower molars corresponds with that of the 

 upper molars. Of the lower incisors the middle and outer pairs 

 (nos. 1 and 3) are soon lost, but the second pair remains functional 

 for a long geological period. When the symphysis becomes short- 

 ened these incisors are sometimes retained as vestiges (e. g. in 

 Mastodon americanus), but in the genus EUplias they have com- 

 pletely disappeared. — Ahstmct of paper read before the Royal Society 

 on March 2i5fh, 1903. 



