270 



ZOOLOGY. 



there existed forms which indicate that the Prohoscidians 

 and Ungulates had a common origin. In the elephants 

 the upper incisor teeth are enormously developed, forming 

 the tusks so characteristic of these animals, while there are 

 none in the lower jaw. There are no canine teeth, while 

 the few molars are large and transversely ridged. In the 

 elephants the ridges on the molar teeth are numerous, the 



Fig. 307.— Section of an elephant's skull, showing: the small size of the brain 

 cavity as compared to the whole skull, and the numerous large air-cells. 

 v, posterior nostrils; 13. cavity of the nose; a. front opening of the bony nos- 

 trils, to the edge of which the' trunk is attached. 



spaces between them being filled with cement. The young 

 mastodon has cement on the upper surface of the tooth; 

 the ridges afterwards become free and covered with enamel. 

 A peculiarity in the elephant's skull is its large size, the 

 brain cavity being very small in proportion to the bulk of 

 the skull itself. To give lightness to what would be other- 

 wise an insupportable weight, the crania} bones contain. 



