readily these sub-classes. They have been called Ornithodelphia, 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 representatives 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 protected 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 Prosimiae), 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 the 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.
