284 AN INTRODUCTION TO ZOOLOGY. 



the Metatheria and Eutheria (Marsupials and higher 

 Mammals), the Marsupial Carnivora being called 

 liapacia, and the others Carnivora. The orders of 

 the Mammalia are therefore founded on many other 

 characters besides those relating to food habits and 

 dentition. 



Teeth of Mammals. — The differences in teeth, 

 which exist in correspondence with differences in 

 habits, although not fundamental characters, are the 

 readiest means of distinofuishing' between mammals of 

 different groups, and they vaiy so much, that they 

 are also the readiest means of distinguishing between 

 different genera.' 



Fig. 107. — Human teeth seen in section, showing the different materials of 

 which they are composed. A^ Pre-moiar ; B, 3ioIar; b, dentine ; c, enamel; a 

 is the pulp-cavity of the tooth. 



The teeth of Mammals consist of three substances : 

 the enamel, which is the thin hard outer coat of 

 the tooth J the dentine, which lies beneath it and 

 forms the major portion of the tooth; and the 

 cement, a tougher substance closely allied to bone, 

 which in human teeth occupies a restricted positiou. 



