GEOGRAPHICAL DISTRIBUTION OF CORALS. %i 



spreading lichen-like. Species of these forms belong to the 

 genus Melobesia. 



They have the aspect of ordinary coral, especially the Mil- 

 lepores, but may be distinguished from these species by their 

 having no cells, not even any of the pin-punctures of those 

 species. 



Besides the more stony kinds, there are delicate species, 

 often jointed, called Corallines, which secrete only a little 

 lime in their tissues, and have a more plant-like look. P>en 

 these grow so abundantly on some coasts, that, when broken 

 up and accumulated along the shore by the sea, they may 

 make thick calcareous deposits. Agassiz has described such 

 beds as having considerable extent in the Florida seas. 



V. THE REEF-FORMING CORALS AND THE CAUSES IN- 

 FLUENCING THEIR GROWTH AND DISTRIBUTION. 



I. Distribution in Latitude. 



Reef-forming species are the warm-water corals of the globe. 

 A general survey of tlie facts connected with the temperature 

 of the ocean in coral-reef seas appears to sustain the con- 

 clusion that they are confined to waters which, through even 

 the coldest winter month, have a mean temperature not below 

 68° F. Under the equator, the surface waters in the hotter 

 part of the ocean have the temperature of 85° F. in the Pacific, 

 and 83" F. in the Atlantic. The range from 68° F. to 85° F. 

 is, therefore, not too great for reef-making species. 



An isothermal line, crossing the ocean where this winter 

 temperature of the sea is experienced, one north of the equa- 

 tor, and another south, bending in its course toward or from 

 the equator, wherever the marine currents change its position, 

 will include all the growing reefs of the world ; and the area 

 of waters may be properly called the coral-reef seas. 



This isothermal boundary line, the isocryme (or cold-water 

 line) of 68° F., extends, through mid-ocean, near the parallel 



G 2 



