372 SYSTEMATIC ZOOLOGY. 



air from tree to tree. Most familiar of all the Australian 

 forms are the large grass-eating kangaroos, in which the 

 fore legs have become almost useless for locomotion, the 

 animal jumping with its hind legs, and, when resting, 

 supporting itself upon these members and its enormously 

 developed tail. There are also fossil marsupials in Aus- 

 tralia, some of them of enormous size. Thus Thylacoleo 

 was as large as a lion, while Diprotodon had a skull three 

 feet in length and a thigh-bone two feet from tip to tip. 



The remaining orders of mammals were formerly placed, 

 in contrast to the Marsupials, as a distinct group, Placen- 

 talia, from the fact that they are not born until their 

 internal organization has been well advanced; and in 

 order that they may be supplied with nourishment a 

 peculiar vascular structure is formed, the placenta, 

 by means of which blood is brought to the growing em- 

 bryo. It has, however, been ascertained recently that 

 some of the Marsupials also have a placenta, and with this 

 discovery the line of course breaks down. 



ORDER II. EDENTATA. 



The edentates, the lowest of the placental mammals, 

 receive their name from the fact that incisor teeth are 

 always lacking, while in the ant-eaters no teeth occur. 

 The feet are armed with strong claws. The group is a 

 tropical one, and has its greatest representation in Amer- 

 ica. Here belong the armadillos, in which the deeper 

 layer of the skin becomes converted into bone, forming 

 an armor over the body. In the fossil Glyptodon this 

 armor formed one solid piece, enclosing the trunk much like 

 the armor of a turtle; but in the living forms it becomes 



