GEOGRAPHICAL DISTRIBUTION OF CORALS. 109 



in greatest profusion and widest variety through its hotter por- 

 tions. Drawing the isocryme of 74° F. (that is, the isotherm 

 for 74° F. as the mean for the coldest month) around the 

 globe, the coral-reef seas are divided, both north and south of 

 the equator, into two regions, a torrid, and a subtorrid, as they 

 are named by the author (see Chart beyond, from the Author's 

 Report on Crustacea) ; and these correspond, as seen below, 

 to a marked difference in the corals which they grow. 



Further, the torrid region should be divided, as the distri- 

 bution of corals show, into a warmer and a cooler torrid, the 

 isocryme separating the two being probably that of 78°. 



But, before considering the facts connected with the geo- 

 graphical distribution of existing coral-reef species, it is impor- 

 tant to have a correct apprehension of what are these reef spe- 

 cies as distinct from those of colder and deeper seas. 



The coral-reef species of corals are the following. — 



1. In the Astraea tribe (Astraaacea), all the many known 

 species. 



2. In the Fungia tribe (Fungacea), almost all known spe- 

 cies, the only exceptions at present known being two free spe- 

 cies found much below coral-reef depths, in the Florida seas, 

 by C. F. de Pourtales, one of them, at a depth of 450 fathoms. 



3. In the Oculina tribe (Oculinacea), all of the Orbicellids ; 

 part of the Oculinids and Stylasterids ; some of the Caryophyl- 

 lids, Astrangids and Stylophorids ; all of the Pocilloporids. 



4. In the Madrepora tribe (Madreporacea), all of the Madre- 

 poricls and Poritids ; many of the Dendrophyllia family or 

 Eupsammids. 



5. Among Alcyonoids, numerous species of the Alcyonium 

 and Gorgonia tribes, and some of the Pennatulacea. 



6. Among Hydroids, the Millepores and Heliopores. 



7. Among Algaa, many Nullipores and Corallines. 



