MAMMALIA 45 1 



lives in water, or burrows in the banks of streams or lakes. It 

 is eighteen or twenty inches long and is covered with soft fur, 

 Its eggs are laid in its burrows. Echidna or the spiny ant-eater, 

 lives in rocky places and captures ants by means of its slender, 

 sticky tongue. They are confined to Australasia and are in- 

 teresting chiefly because of their likeness to the reptiles and 

 birds (Pig. 238). 



Subclass II. Eutheria: Division I, Didelphia, Order Mar- 

 supialia. — Mammals with nipples; these occur in the pouch on 

 the ventral surface of the body in which the immature young 

 are placed at birth. The young are too immature to suck vol- 

 untarily at first, and milk is forced into the mouth by the action 

 of muscles about the gland. The pouch is usually supported by 

 two bones attached to the pubis and extending forward. There 

 are two oviducts, two uteri, and even the vaginae may be paired 

 (Figs. 52, 62). 



Many different types are included in this group. Some are 

 rat-like in appearance, others similar to the dog, others to the 

 bear. Some are herbivorous, some carnivorous, others insec- 

 tivorous. With the exception of the American opossum fam- 

 ily, the living species are native of Australasia. Fossil marsupials 

 are found in all parts of the world, showing that they are an 

 ancient type of mammals which have become extinct except in 

 the places cited. Many of the fossil forms were of gigantic size. 

 The largest living species is the kangaroo. 



Division II. Monodelphia {one uterus), Order Placentalia — 

 Mammals in which the young are connected to the wall of the 

 maternal uterus by means of a placenta (see §462) ; two oviducts; 

 uteri more or less united into one; vagina single; no cloaca; no 

 marsupium. The segmentation of the ovum is total. 



The following key will assist the student to get a view of 

 the principal orders of the placental mammals : 



Teeth wanting, or without enamel Edentata. 



Teeth with enamel. 



Hind limbs wanting. 



Front appendages with elbow joint Sirenia. 



Front appendages without elbow joint Cetacea. 



