THE TERTIARY PERIOD 325 



velopment of pillar-like limbs to support the enormous weight. 

 Increase in size and complexity of the teeth and their consequent 

 diminution in numbers and the development of the peculiar method 

 of tooth succession. The loss of the canines and of all of the incisor 

 teeth except the second pair in the upper and lower jaws and the 

 development of these as tusks. The gradual elongation of the 

 symphysis or union of the lower jaws to strengthen and support 

 the lower tusks while digging, culminating in Tetrabeledon (or 

 Gomphotherium) Angustidens. The apparently sudden shortening 

 of this symphysis following the loss of the lower tusks and the com- 

 pensating increase in size and the change in curvature of those of 

 the upper jaw. 



" The increase in bulk and height, together with the shortening 

 of the neck necessitated by the increasing weight of the head with 

 its great battery of tusks, necessitated the development of a pre- 

 hensile upper lip which gradually evolved into a proboscis for food 

 gathering. The elongation of the lower jaw implies a similar 

 elongation of this proboscis in order that the latter may reach 

 beyond the tusks. The trunk did not, however, reach maximum 

 utility until the shortening jaw, removing the support from be- 

 neath, left it pendant, as in the living Elephant." 1 



Carnivores (Tigers, Dogs, etc.). These modern flesh-eaters 

 can be traced back to a generalized order or group (so-called 

 Creodonts, Fig. 207) which had certain characters suggesting the 

 Insect-eaters, hoofed Mammals, and Marsupials, as well as the 

 Carnivores. These Creodonts or ancestral flesh-eaters had small, 

 simple brains and many small teeth. In the course of evolution 

 the existing carnivorous families have been derived from them. 



Rodents (Rats, Porcupines, Squirrels, etc.). The Rodents 

 (gnawers) can be traced back to the early Eocene, when the incisor 

 teeth were just developing a structure suitable for gnawing. By 

 the middle of the Eocene the Rodents were common and their 

 incisors were highly specialized for gnawing. Primitive Squirrel- 

 like forms are known from the late Eocene. 



Insectivores (e.g. Moles, Hedgehogs, etc.). These have also 

 been traced back to the Eocene, and, like the Rodents, they still 

 show many of their ancestral or primitive features. They have 

 changed much less than most of the other classes of Mammals. 



Cetaceans (e.g. Whales, Porpoises, etc.). In our study of 

 1 R. S. Lull: Amer. Jour. Sci., Vol. 25, 1908, pp. 11-13. 



