GEOGRAPHICAL DISTRIBUTION. 269 



tropical regions, tlie mean winter temperature of the water is 

 not below 68° F. 



Passing from the Indian to the Atlantic Ocean, we find little 

 or no coral on the west coast of Africa. The islands of Cape 

 St. Ann and Sherboro, south of Sierra-Leone, are described as 

 coral by Captain Owen, R.N., in the Journal of the Geographi- 

 cal Society (vol. ii., p. 89) ; but this has been since denied. 

 The Island of Ascension, in 7° 56' S., and 14° 16' W., must 

 have been bordered by growing coral, as Quoy and Gaymard 

 mention that a bed of coral rock may be seen buried beneath 

 streams of lava. Quoy also states that the corals which formed 

 these reefs are no longer found alive, and adds that volcanic 

 eruptions have probably destroyed them. The cold polar 

 currents along the western African coast are the cause of the 

 absence of corals from it, to within six or seven degrees of the 

 equator ; and these cold waters may at times extend still farther 

 north. The same obstacle to the diffusion of species east- 

 ward, mentioned as occurring in the Pacific — that is, westward 

 currents — exists also in the Atlantic, and probably with the 

 same effect. 



On the American shores of the Atlantic, north of the 

 equator, there are few reefs, except in the West Indies. The 

 waters of the Orinoco and Amazon, and the alluvial shores they 

 occasion, exclude corals from that part of the coast. 



In the West Indies, the reefs of Florida (p. 1 67), Cuba, the 

 Bahamas (p. 174), and of many of the eastern islands, are well 

 known. On the east coast of Florida they continue up as far 

 as Cape Florida, in latitude 25° 40' N. ; but the west coast is 

 free from them. There are also said to be patches at intervals 

 along the coast of Venezuela and Guatemala; but the west 

 shores of the Gulf of Mexico, as well as the northern, hke West 

 Florida, are mostly low, and without reefs ; they are within the 

 influence of the Mississippi and other large rivers. Some 

 species of reef corals however occur in the vicinity of 

 Aspinwall (p. 87). The Bermudas are of coral origin, and 

 the most northern point of growing reefs. 



South of the equator, on the east coast of South America, 



