564 



ZOOLOGY 



SECT. 



remains enclosed in the marsupium till it reaches an advanced 

 stage of development. Ornithorhynchus develops no marsupium, 



and the two eggs which it pro- 

 duces are deposited in its bur- 

 row. In Echidna the egg-shell 

 is composed of keratin; in 

 Ornithorhynchus, it contains 

 carbonate of lime. The ova of 

 the Prototheria are very much 

 larger than those of other Mam- 

 mals (Fig. 1157), their large 

 dimensions being due to the 

 presence of a large proportion 

 of food yolk. The segmentation, 

 unlike 'that of all the Theria, 

 with the exception of the Koala, 

 is meroblastic. 



Distribution. The Mono- 

 tremes are entirely confined to 

 Australia, Tasmania, and New 

 Guinea. The Marsupials are 

 most abundantly represented 

 in the Australian region, the 

 greater number of the Australian 

 families and genera being re- 

 stricted to the Australian con- 

 tinent, and to Tasmania, though 



several genera extend to New Guinea and some of the neigh- 

 bouring islands. The family Didelphyidas, or Opossums, inhabits 

 South America and extends into the southern part of North 

 America; and a single genus Ccenolestes of a family related to 

 the Australian Diprotodonts has been recently found in South 

 America. 



The Edentates are most numerously represented in South and 

 Central America, the true Ant-eaters, the Sloths, and the Arma- 

 dillos being all inhabitants of that region. But the Scaly Ant- 

 eaters and the Ard-varks (Cape Ant-eaters) are denizens of the 

 Old World ; the former inhabiting Southern Africa and South- 

 Eastern Asia, the latter being confined to Africa. The Cetacea 

 are cosmopolitan in their distribution: the great majority are 

 marine, but some ascend rivers, and a few are exclusively fluviatile, 

 inhabiting the rivers of South America and South-Eastern Asia. 



The distribution of the Sirenia is somewhat restricted. The 

 recently . extinct Rhytina inhabited Behring's Straits. The 

 Manatee is confined to the Atlantic coasts of South America and 

 of Africa, living chiefly in the larger rivers. The Dugong occurs 

 on the east coast of Africa, in the Ked Sea, the Indo-Malayan 



FIG. 1157. A, blastula stage of one of the 

 Theria. B, transition stage between 

 the niorula and blastula in a Memo- 

 ir erne. Both represented in diagram- 

 matic section. (After Semon.) 



