84 CO.?.^/.S AND CORAL ISLANDS. 



of 28°; but in the vicinity of the continents it varies greatly 

 from this, as explained beyond in the course of remarks on 

 the geographical distribution of reefs. It is to be observed 

 that the temperature of 68° F. is a temporary extreme — not 

 that under which the polyps will flourish: Except for a short 

 period, the waters near the limits of the coral seas are much 

 warmer; the mean for the year is about 731" F. in the North 

 Pacific, and 70° F. in the South; from which it maybe in- 

 ferred that the summer mean would be as high at least as 78° 

 and 74° F. 



Over the sea thus limited coral reefs grow luxuriantly, yet 

 in greatest profusion and widest variety through its hotter 

 portions. Drawing the isocryme of 74° F. (that is, the iso- 

 therm 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 distribu- 

 tion 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 

 important to have a correct apprehension of what are these 

 reef species as distinct from those of colder and deeper seas. 



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



1. In the Astraea tribe (Astrseacea), ail the many known 

 species. 



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

 the only exceptions at present known being two free species 

 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 Caryo- 

 phyllids, Astrangids and Stylophorids ; all of the Pocillo- 

 porids. 



