238 POUCHED MAMMALS. 
Placental Mammals, thus showing that these islands have had some connection 
with those of the Malayan region. The channel separating Lombok and Celebes 
from Java and Borneo is, however, of much greater depth than those dividing the 
other islands; and it may accordingly be inferred that any land-connection which 
formerly existed between the two groups must have been remote. 
In the Tertiary period opossums were distributed over a large portion of 
Kurope; and recent discoveries indicate the existence during the same epoch of 
marsupials allied to the Australian thylacine in South America. In the preceding 
Secondary period, as we shall show later on, Pouched Marsupials appear to have 
ranged over the whole world, and were then, together with Egg-laying Mammals, 
the chief, if not the sole representatives of the class. 
THE KANGAROOS AND THEIR ALLIES. 
Family J/4CROPODID.Z. 
We commence our survey of the Pouched Mammals with their most aberrant 
and specialised representatives, or those commonly known as kangaroos, wallabies, 
rat-kangaroos, etc. And it may 
be mentioned here, that whereas 
kangaroos are very frequently 
spoken of as typical Marsupials, 
this is really very far from being 
the case. It is true that they con- 
form in all essential characteristics 
to the Marsupial type of structure ; 
but they have been specially 
modified for a particular kind 
of progression — namely, leaping. 
This has profoundly modified their 
whole organisation, and rendered 
them some of the most specialised 
of all Mammals; and they retain 
accordingly but little resemblance 
to what may be termed a typical, 
or generalised marsupial, such as 
a bandicoot, or thylacine. 
The kangaroos belong to a 
group or suborder of Marsupials 
characterised by the adaptation of 
their teeth to a vegetable diet. 
Thus the front, or incisor teeth, 
as shown in the figure on p. 239, 
are never more than three in 
number on each side of the jaws, and are usually three in the upper and one in 
the lower jaw; while in all cases the innermost pair in each jaw are of large size, 

SKELETON OF KANGAROO, 
