44 ZOOLOGY. 



Subclass II. Didelphia or Marsupialia. 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 voluntarily 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 running forward. There are two oviducts, 

 two uteri, and even the vaginae may be paired (Figs. 50, 60). 

 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 mar- 

 supials 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. 



Subclass III. Monodclphia or Placentalia. Mammals in 

 which the young are connected to the wall of the maternal 

 uterus by means of a placenta (see 451 ) ; 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. 



Hind limbs developed. 



Nails of the digits hoof-like Ungulata. 



Nails claw-like. 



The front limbs modified to form wings, 



Cheiroptera. 

 No wings. 



Thumbs not opposable. 



Incisors and canines small, 



Insectivora. 



