1 66 ANIMAL LIFE PAST AND PRESENT. 



worthy that two condyles are found in the Amphibians 

 (Frogs, Newts, &c.).* In their warm blood these 

 creatures also agree with other mammals ; but this is 

 a feature of no importance from a classificatory point 

 of view, since it is found in Birds and Mammals which 

 we must regard as being independently derived from 

 cold-blooded animals. Again, although the Duck-bill 

 and the Echidna, as necessarily follows from their warm 

 blood, have a four-chambered heart like all other 

 mammals, yet it is noteworthy that the heart contains 

 certain peculiarities connecting it in some degree with 

 the three- chambered heart of Eeptiles. These animals 

 differ, however, from all other mammals in that the 

 tubes for carrying off the waste products of the body 

 have but a single common aperture, precisely as in 

 Birds and Reptiles, and they are therefore generally de- 

 scribed as the Monotremes, or Monotremata, by which 

 name we shall henceforth refer to them. 



Two other remarkable peculiarities connected with 

 the skeleton require notice in some detail. Exclusive 

 of the Monotremes, all mammals, without exception, 

 have only two bones on either side of the body by 

 means of which the arm is connected with the trunk, 

 and which are collectively spoken of as the shoulder- 

 girdle. These bones are respectively the shoulder-blade 

 at the back, and the collar-bone in front ; the head of 

 the arm-bone being received into a cup-like cavity in 

 the lower end of the shoulder-blade. Now in the 

 Monotremes we find, in addition to the shoulder-blade, 



* See page 67. 



