236 POUCHED MAMMALS. 



animated lumpa Previous to birth there is no placental communication between 

 the blood-vessels of the foetus and the parent ; and at birth the rudimental young 

 are transferred to the teats of the female, to which they adhere tightly for a long 

 period, their lips being specially modified into a cylindrical sucking-organ. In 

 most case3 the young thus suspended are protected by a fold of skin on the 

 abdomen of the female, which forms a pouch in which the teats are contained. 

 From the universal absence of a placenta, these Mammals are regarded as forming 

 a subclass of equal rank with the Placentals or Eutherians, and are spoken of as 

 Implacentals or Metatherians ; the latter term indicating their lower position, as 

 compared with the Eutherian, or highest Mammals. 



The Placental, or Eutherian Mammals are, as we have seen, divided into 

 numerous orders; and it may be thought that similar divisions could be instituted 

 among the Implacentals or Metatherians. It happens, however, — so far at least 

 as existing forms are concerned, — that this is not the case ; but so as to render 

 our classification symmetrical, it is necessary to have a name for the one order of 

 Implacentals, the term Pouched Mammals, or Marsupials, has been selected, and we 

 shall speak of these Mammals under either of these terms ; it must, however, be 

 constantly borne in mind that they also have the higher designation of Implacentals. 

 or Metatherians, ranking with the term Placentals, or Eutherians. 



In addition to the primary distinction of the absence of a placenta, the 



Pouched Mammals present certain other more or less distinctive peculiarities. 



Mention has already been made of the general presence of a pouch, or marsupium, 



in which the abdominally-placed teats of the female are concealed ; and to this it 



may be added that, with the single exception of the thylacine, the front brim of 



the pelvis always has a pair of divergent splint-like bones projecting forwards in 



the form of the letter Y. These so-called marsupial bones — shown in all our 



figures of the skeletons of this group — were originally considered to be for the 



purpose of affording support to the pouch; but this view is discredited by their 



presence in both sexes. A peculiarity of the skull of all Pouched Mammals save 



one, is that the so-called angle, or lower posterior projection of the lower jaw, is 



more or less bent inwards, or inflected, as seen in the figure of the skull of Gray's 



rat-kangaroo, given in the sequel. This peculiarity is not, however, distinctive of 



the order, since it also occurs in some of the Insectivores. The skull of every 



marsupial is further characterised by the presence of larger or smaller vacuities, or 



unossified spaces, in the bony palate. As regards their brains, it may be observed 



that all the Pouched Mammals display a low grade of organisation ; the whole 



brain being small in proportion to the size of the body, while the foldings on the 



surface of its hemispheres are never of a very complex nature, and only developed 



at all in the largest members of the order. The reproductive organs of the female 



are likewise constructed after a lowly fashion ; the oviducts always remaining 



perfectty separate from one another, and never uniting, as they do in so many of 



the Eutherian Mammals, to form a common chamber, or womb. 



Certain peculiarities connected with the number and mode of 

 Teeth. 



replacement of the teeth also aid in distinguishing marsupials from 



other Mammals. In the first place, as shown in the figure of the skull of the 



Tasmanian devil given later on, there may be more than three pairs of front or 



