UNGULATA. 301 



the upper incisors, which are persistently growing and only 

 enamelled at the extreme apex, which soon wears away. As to 

 the grinding teeth (fig. 171), those of Dinotherium are compara- 

 tively small, with never more than two or three transverse 

 ridges, which are about as deep as thick, invested with enamel, 

 and separated by a deep valley (A) ; the molars of Mastodon 

 have slightly more numerous and somewhat deeper ridges, but 

 these are also separated by deep valleys (B) ; while the grinding 

 teeth of Elephas exhibit still more numerous and deepened 

 ridges (now to be described as "plates"), separated by cleft- 

 like valleys, which are filled to overflowing by soft bony cement 

 (c, D). As the complication of the grinding teeth and their 

 relative increase in size take place, the successional teeth are 

 gradually lost, and the entire series of grinding teeth cannot be 

 accommodated in the mouth at one time. Thus arises the 

 remarkable mode of development observed in the existing 

 elephant, where rarely more than two never more than parts 

 of three grinders are functional in each half of either jaw at 

 one and the same time, and a series of six or seven molars 

 succeed each other from behind forwards during the life of the 

 animal, the new ones emerging from a socket at the back of 

 the jaw while the worn teeth are crushed out of the mouth in 

 front. A somewhat similar mode of succession is to be noted 

 in the kangaroo and the manatee. 



While this process of evolution in the teeth has occurred, 

 there is also some reason to believe that the proboscis has 

 increased in size ; for the skull in some of the earlier mastodons 

 is much less thickened by the growth of cellular bony tissue 

 than that of the later mastodons and elephants, and there 

 would thus be less surface available for the necessary muscular 

 attachments in the earlier than in the later forms. 



Dinotherium (figs. 171 A, 172, 173). Only one imperfect example of 

 the skull of Dinotlierium is known, but many fine specimens of the 

 mandible and the dentition have been discovered. The cranium (fig. 172) 

 is long and low compared with that of the elephant, and it seems to taper 

 in front without space for upper incisors, though this appearance may be 

 accidental and due to fracture. The slender mandibular raini are fused 

 together at the symphysis, where they are bent downwards and bear a 



