270 
ZOOLOGY. 
there existed forms which indicate that the Proboscidians 
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 ujiper surface of tlie 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 1o the bulk of 
the skull itself. To give lightness to what would be other- 
y^i^Q an insupportable weight, the cranial bones contain 
