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AN INTRODUCTION TO ZOOLOGY. 



tivity, their teeth, grow inconveniently long, through 

 want of use, and may even cause the death of the 

 animal by penetrating the opposite jaw. Hence a 

 tame rat, if not allowed at large, should be supplied 

 with a piece of wood to gnaw at. The outside edge of 



Fig. 121.— Kodent dentition al a Marsupial, the Wombat, PIuiscalom]/s ITombat. 

 Dental Formula, — — _; incisors and molars both growing from persistent 

 pulps. .4, upper jaw; B, lower jaw; C, single molar. 



these incisors is protected by hard enamel, while the 

 inside edge is not ; hence the inner edge wears down 

 first, and thus the outside edge is always kept very 

 sharp. Similar sharp edges of enamel are maintained 

 on the molars by the presence of edges of enamel. 



