CHAPTER V
THE EXISTING ORDERS OF MAMMALS
Prototheria—Monotremata
Apart from those creatures 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 Reptilia, 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 Triconodonta, 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,
