THE CLASS BIRDS. 



279 



ductive organs present many anomalies. The dental system 

 is rudimentary, and in some a horny covering of the jaws 



Fig. 220. The Ornithorhynchus. 



gives to them the appearance of a duck's bill, 

 two genera of this singular class is known ; 

 rhynchus (Fig. 220), and the echidna. 



As yet only 

 the ornitho- 



THE CLASS BIRDS. 



426. The class birds is one of the best defined and most 

 distinct, whether viewed with reference to the exterior or 

 interior. Birds are oviparous vertebrate animals, with a 

 double and complete circulation; to which may be added, that 

 the respiration is aerien and double; which means, that instead 

 of being confined to the lungs, as in mammals, the air pene- 

 trates throughout the body and even into the interior of the 

 bones ; their blood is hot, as in mammals. Finally, they are 

 covered with feathers, and their pectoral extremities have the 

 form and character of wings. 



