28o AN INTRODUCTION TO ZOOLOGY. 



Mammalia. 



Link between Mammalia and Reptiles. — 



The Mammalia, or animals that suckle their young, 

 are connected with Reptiles by the existence of a link 

 between them, the Australian animal called the duck- 

 billed mole, or Ornithorhynchus (bird- billed animal). 



IPig. 105. — The Duct-billed Mole, OrnithorhyneJius paradoxus, a link between 

 Reptiles and Mammalia. 



To this animal, with another similar kind, called 

 Echidna, the group name of Monoteemata has been 

 given. The structure of its pectoral girdle is like 

 that of a reptile, and, like a reptile, it lays eggs. 



Characters of the Mammalia. — The Mammalia, 

 as already stated, are the only vertebrates which have 

 non-nucleated red blood corpuscles. The blood is 

 warm. Another general character is that they have 

 teeth which are placed in sockets, and usually de- 

 veloped in two series, the first, or milk teeth, falling 

 out when the others push them out by their growth. 

 Besides teeth developed in sockets, another universal 

 characteristic is the pi-esence of hair on the skin, 

 which is never entirely absent, and in the vast majority 



