426 



METAZOAN PHYLA 



which is bony in character and derived from the dermis. The dentine 

 is covered over the crown with a layer of enamel, which is derived from 

 certain epitheUal cells, and around the root by cementum, deposited 

 by the dermis after the dentine has been formed. In the center of the 

 tooth is a pulp cavity of soft connective tissue provided with blood vessels 

 and nerves. 



Fig. 317. — Types of mammalian skulls, showing character of dentition. From speci- 

 mens. In each case the anterior part of the skull is shown from the side and one-half of 

 the same part from below, with the lower jaw removed. A, European hedgehog, an 

 insectivore. X %• B, coyote, or prairie wolf, a carnivore. X }i- C, beaver, a rodent. 

 X %. D, sheep, an ungulate. X M. 



The aUmentary canal shows several distinguishing characteristics. 

 The mouth cavity is separated from the nasal chambers by a hard palate, 

 which is a shelf of bone covered by soft tissues (Fig. 232). This is 

 supplemented posteriorly by a fleshy soft palate. The passage from the 

 mouth into the pharynx is known as the fauces, on each side of which 

 lie the tonsils. The latter are masses of lymphoid tissue and their 



