150 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



sign is used to represent the disposition in the upper 

 and lower jaws. Making use of this method of 

 formulation, we may represent the typical dentition 

 of a heterodont mammal thus : 



. 3.3 1.1 4.4 3.3 



t , c , nm , 7 = 44. 

 3.3 1.1' " 4.4' 3.3 



This is the dental formula of the low insectivorous 

 mammal Gymnura. 



The dental formula of man is : 



2.2 1.1 2.2 3.3 



And that of the cat : 



3.3 1.1 3.3 1.1 



s73 c TTi pm 272 m i7i = 30< 



There are a number of certain modifications in 

 the form or structure of the tooth which at once 

 attract attention. While those Mammals, such as man, 

 which bite their food have sharp incisors, those that 

 gnaw it have the greater part of the surface of the 

 incisors devoid of that hardest part of the tooth 

 which is called the enamel, and so maintain an edge, 

 the softer dentine always wearing down faster than 

 the enamel which is at the front and sides. The 

 teeth which are set at the outer angles of the jaw 

 (the canines) are especially large in those forms which 

 seize on a living, and require to hold a struggling 

 prey \ and it is these organs which are most frequently 

 converted into weapons of attack, though in the most 

 prominent case of all, that of the elephant, the tusks 

 belong to the incisor series. In the male boars 

 (Suidse), the canines attain to a considerable length, 

 being in the Babirussa, an animal not so large as the 



