70 EVOLUTION OF (Lite, 
readily these sub-classes. They have been called Ornitho- 
delphia, Didelphia, Monodelphia. The first sub-class is 
so called from the terminal arrangement of the abdominal 
viscera being the same as that of Birds and Reptiles. 
The Ornithorhynchus and Echidna are the only represent- 
atives of the Ornithodelphia, and are limited to Australia 
and Tasmania. They seem to be the survivors of a class 
once much larger, and now extinct. The Didelphia include 
the Kangaroos (Fig. 80), Wombats, Opossums, etc. With 
the exception of the Opossums, they are also confined to 
Australia and the adjacent isles. The most striking feature 
in this sub-class is the pouch in which the young are pro- 
tected in their helpless condition. The third sub-class of 
Mammalia, the Monodelphia, contains as many as twelve 
orders, of which the following will serve as examples: Dog, 
Whale, Lemur (a sort of Monkey, one of the Prosimiz), 
Ape, Man, Beaver, Hyrax, Elephant, Bat, Hedgehog, Sloth, 
Horse, Pig, and Sea-cow. The names of the orders of 
which these animals are examples may be seen in the 
accompanying diagram. It is to be understood that there 
are many other examples of each order, which want of 
space prevents us from inserting. Of all Mammalia, the 
Ornithorhynchus (Fig. 79) and Echidna approach nearest 
the Birds and Reptiles, not only in the characteristic 
arrangement of the abdominal viscera, but also in the 
skeleton. The collar-bone (clavicle), the breast-bone 
(sternum), and the coracoid process of the shoulder-blade 
(scapula) form together a fork-shaped bone similar to that 
of Birds and Lizards. This fork-shaped bone is not present 
in the other Mammalia. The ribs in the Ornithorhynchus 
offer thé same arrangement as seen in the Crocodile, while 
the skull is very bird-like in the articulations of its bones, 
and in the arrangement of the organs of hearing (semi- 
circular canals) and the nerve of smell. 
While, therefore, there can be little doubt that the class 
-- — MUR 
