6 Mammals of Eastern Asia 
stabbing or nipping technique. The central lower incisors become 
enlarged and point forward from the front of the jaw, while 
the usually large canines remain rather small or become very 
small. This condition appears in the Shrews and the South 
American Csenolestids. It is possible that the ancestral Rabbits, 
rodents, Phalangers, and Kangaroos used their centrally placed 
incisors in this way before they became so highly vegetarian. 
When carnivorous animals took to living permanently in the 
water, wholly new things happened to their teeth. In some, the 
Seals and Whales, a tendency toward uniformity of shape plus 
simplification of structure in the premolars and molars ap- 
peared. The cheek teeth, especially in the Toothed Whales, be- 
came peglike. Although the number of the cheek teeth in some 
of the Toothed Whales tended to increase, in some Ziphioid 
Toothed Whales and all Whalebone Whales the teeth tended 
toward or achieved obsolescence. 
In mammals with omnivorous diets, such as some Primates, 
Bears, Raccoons, Pigs, Murinse, certain Squirrels, and certain 
of the Marsupial Phalangers, the cheek teeth may become en- 
larged and bear on their crown surfaces mound-like cusps that 
both crush and grind. 
An almost completely vegetarian diet, especially if it consists 
chiefly of leaves of trees and grasses, requires an elaborate 
mechanism for chopping and grinding the relatively resistant 
cellulose. This is provided by more or less complicated folding 
of the lateral walls of the teeth to produce raised surface plates 
or meandering patterns of hard enamel on the crowns, inter- 
spersed by depressed softer areas of cement. The different 
degrees of wear in these two substances maintain the sharp, 
file-like surface seen in the molars of many ungulates, Kanga- 
roos, and Langur Monkeys. In the ungulates the canines be- 
come similar to the incisors or disappear, and the premolars take 
the form and function of the molars and act with them. 
The folded molar pattern in browsing and leaf-eating mam- 
