EQUATORIAL ZONE. 219 



Africa the desert of the Sahara forms a well-marked boundary 

 between the equatorial and northern zones ; as the boundary 

 approaches the Nile it makes a sudden sweep towards the 

 north as far as Northern Syria (Mastacemhelus, near Aleppo, 

 and in the Tigris ; Clarias and Chromidcs, in the lake of 

 Galilee) ; crosses through Persia and Afghanistan (Ophioce- 

 phalus), to the southern ranges of the Himalayas, and follows 

 the course of the Yang-tse-Kiang, which receives its con- 

 tingent of equatorial fishes through its southern tributaries. 

 Its continuation through the North Pacific may be considered 

 to be indicated by the tropic which strikes the coast of 

 Mexico at the southern end of the Gulf of California. Equa- 

 torial types of South America are known to extend so far 

 northwards ; and by following the same line the West India 

 Islands are naturally included in this zone. 



Towards the south the equatorial zone embraces the 

 whole of Africa and Madagascar, and seems to extend still 

 farther south in Australia, its boundary probably following 

 the southern coast of that continent ; the detailed distribu- 

 tion of the freshwater fishes of South-Western Australia has 

 been but little studied, but the few facts which we know show 

 that the tropical fishes of Queensland follow the principal 

 water-course of that country, the Murray Eiver, far towards 

 the south and probably to its mouth. The boundary-line 

 then stretches northwards of Tasmania and New Zealand, 

 coinciding with the tropic until it strikes the western slope 

 of the Andes, on the South American Continent, where it 

 again bends southwards to embrace the system of the Eio de 

 la Plata. 



The equatorial zone is divided into four regions : — 



A. The Indian region. 



B. The African region. 



C. The Tropical American region. 



D. The Tropical Pacific region. 



