THE ANIMAL KINGDOM 



27 



Ornithorhynchus, the duck-bill of Australia, is the best-known genus; 

 and there are two species of the spiny anteater, echidna. There seem to 

 be no more than a half-dozen species surviving for the entire sub-class. 



ORNITHORHYNCHUS 



Fig. 21. — Ornithorhynchus is a repre- Fig. 22. — Opossum, the typical genus of 

 sentative of the most primitive group of Didelphians. (Redrawn after Newman.) 

 mammals, the Monotremes. As an 

 egg-laying mammal it bridges over the 

 gulf separating reptiles and mammals. 



Sub-Class Marsupials 



The marsupials or didelphia give birth to their young in a most imma- 

 ture state and nourish them for some time in an external marsupial pouch 

 situated on the ventral side of the body of the 

 female. The brain has no corpus callosum. 

 A loose allantoic placenta occurs in some. 

 Dasyurus has a yolk-sac placenta. 



Opossum and kangaroo are well-known 

 examples. All the indigenous mammals of 

 Australia are non-placental. 



Sub-Class Placentalia 



The placentals or monodelphia have a 

 placenta, a corpus callosum in the brain, and 

 no marsupial bones. Urogenital and digestive 

 outlets are separated. 



Placentals are subdivided into at least ten 

 living orders. 



Order I. Insectivora. The insectivores 

 include shrews, moles, and hedgehogs. They 

 are flat-footed and five-toed, and their denti- 

 tion is also unspecialized, so that they are 

 apparently nearest of surviving forms to the 

 original placental. 



Order 2. Xenarthra. The xenarthra include part of the group 

 formerly included in the edentates such as the armadillos, sloths and 



TUPAIA-AN INSECTIVORE. 



Fig. 23. — Tupaia, the tree- 

 shrew, an insectivore. 



