CHAPTER V 



THE EXISTING OllDERS OF MAMMALS 

 PROTOTIJERIA MONOTREMA TA 



APART from those creatbres whose fragmentary remains have 

 been considered in the last chapter, and which belong to the 

 earliest of mammaliferous strata, the remains of Mammalia are 

 all referable to existing orders. In the pages which follow we 

 shall therefore deal with the actual representatives of living 

 families side by side with their extinct relatives. The existing 

 orders of Mammalia, together with those of their fossil allies, can be 

 plainly divided into two great subdivisions, or, as we shall term 

 them, sub-classes ; the Mammalia as a whole being termed a class 

 of the Vertebrata comparable with the class Eeptilia, etc. It 

 has been usual, owing to the initiative of Professor Huxley, to 

 divide the Mammalia into three divisions of primary importance. 

 We shall adduce reasons later for not accepting this mode of 

 division, but that which allows of only two primary divisions. 

 These two divisions are (1) Prototheria and (2) Eutheria. 

 Whether the Multituberculata, Trituberculata, and Tricono- 

 donta, considered in the last chapter, are really to be distributed 

 among these two sub-classes is a matter upon which it is possible 

 to form an opinion, but not to dogmatise. The Prototheria 

 stand at the base of the mammalian series, and present many 

 likenesses to the Sauropsida ; the Eutheria are the animals 

 which are most fully differentiated as mammals. We shall 

 commence with 



SUB-CLASS I. PROTOTHERIA. 



To this group belongs the order Monotremata, and possibly 

 also the so-called Allotheria or Multituberculata. As, however, 



