VERTEBRATA. 



297 



but the molars are sufficiently characteristic. The 

 teeth of insectivorous Bats, and of insectivorous Mar- 

 supials, resemble in type the teeth of the order Insec- 

 tivora (hedgehogs, moles, and shrew mice). 



Dentition of Rodents. — A very interesting type 

 of dentition is that of the Rodentia, characterized 

 by the great development of the incisors. These 

 grow from permanent pulps ; that is to say, they 



Kg. 120.— ^, Skull of Kat, with sharp-edged incisors, and molars g^jj. B, 



Molars of Black Bafc (Jlfu« rattus), four times natural size, showing enamel 

 ridges left by the wearing down of the teeth. 



keep on growing all the time, an arrangement which 

 provides for the great wearing down of the teeth 

 resulting from the gnawing habits of the animals, 

 from which they receive their name. One conse- 

 quence of this is, that if rats, e.g., are kept in cap- 



