PHYLUM CHORD AfA 



570 



eventually forms a complete investment, of two layers, to the yolk. 

 An embryonic area is differentiated at one pole, and on it appears 

 a primitive streak with a primitive knot and head-process. 



Geographical Distribution. The Monotremes are entirely 

 confined to Australia, Tasmania, and New Guinea. The Mar- 

 supials are most abundantly represented in the Australian 

 region, the greater number of the Australian families and 

 genera being restricted to the Australian continent and to 

 Tasmania, though several genera extend to New Guinea and some 

 of the neighbouring islands. The family Didelphyidse, or Opos- 

 sums, inhabits South America and extends into the southern 

 part of North America ; and a single genus, Cmnolestes, of a 



family at one time supposed to be 

 related to the Australian Diproto- 

 donts, but now more generally 

 regarded as derived from Didel- 

 phyid (Polyprotodont) stock, has 

 been comparatively recently found 

 in South America. 



The Edentates are most numer- 

 ously represented in South and 

 Central America, the true Ant- 

 v eaters, the Sloths, and the Arma- 

 \ dillos being all inhabitants of 

 \ that region. But the Scaly Ant- 

 | eaters and the Ard-varks (Cape 

 I Ant-eaters) are denizens of the 

 / Old World ; the former inhabiting 

 Southern Africa and South-Eastern 

 Asia, the latter being confined to 

 Africa. 

 FIG. 1288.-J, biastuia stage of one of the The . Cetacea are cosmopolitan 



Theria. J5, transition stage, between in their distribution I the great 



the morula and biastuia in a Mono- . , 



treme. Both represented in diagram- ma]Onty are marine, but SOUie 



ascend rivers, and a few are ex- 

 clusively fluviatile, inhabiting the rivers of South America and 

 South-Eastern Asia. 



The distribution of the Sirenia is somewhat restricted. The 

 recently extinct RJiytina 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 Red Sea, the Indo-Malayan 

 islands, and the northern coast of Australia. 



The Ungulata occur in all the great regions, with the 

 exception of the New Zealand, Polynesian, and Australian. 

 Oxen are, with the exception of the American Bison, natives of 

 the Palaearctic, Ethiopian, and Oriental regions. Wild sheep, 



