112 COMPARATIVE DENTAL ANATOMY 



The skull is solidly combined in one piece (Fig. 

 37). The oral armature is remarkable in that 

 there are no teeth whatever, but the maxillaries 

 and mandible are inclosed in hard, bony cases 

 which form beaks like the strong bills of birds. 

 These beaks are variously formed, being in a gen- 

 eral way sharp and thin-edged in the carnivorous 

 species and adapted to cutting, and blunt and flat 

 in the herbivorous types and adapted to crushing 

 vegetable substances. 



The fourth class of Vertebrates, the Birds 

 (Aves), are warm-blooded and clothed with feath- 

 ers, which by special modification and general 

 structure enable them to fly through the air. 

 Birds are closely related to the reptiles in many 

 respects, and are probably descended from a com- 

 mon ancestral form with them. 



This class is unique in that none of its living 

 members are provided with teeth, but instead of 

 which, the elongated maxillae and mandible are 

 covered with horny structures called bills or 

 beaks, which take the place of teeth and perform 

 the dental offices. The beak is the prehensile and 

 cutting organ, and is variously modified to adapt 

 it to the different kinds of food employed. Mas- 

 tication, when necessary, is performed by the 

 gizzard, a strong muscular stomach. The beak is 

 short and strong in the grain-feeders; long and 

 slender in the insect-eaters; short and gaping in 



