illustrating the Distribution of Marine Animals. 193 



we have laid down are not, however, those of any chart pre- 

 viously constructed, for the reason stated, that they mark the 

 positions where a given temperature is the mean of the 

 coldest month (or coldest thirty consecutive days) of the 

 year, instead of those where this temperature is the mean 

 annual or monthly heat; and hence the apparent discre- 

 pancies which may be observed, on comparing it with iso- 

 thermal charts. 



The isocrymal lines adopted for the chart are those of 80°, 

 74°, 68°, 62°, 56°, 50°, 44°, and 35° of Fahrenheit. The tem- 

 peratures diminish by 6°, excepting the last, which is 9° less 

 than 44°. 



In adopting these lines in preference to those of other de- 

 grees of temperature, we have been guided, in the first place, 

 by the great fact, that the isocryme of 68° is the boundary- 

 line of the coral-reef seas, as explained by the author in his 

 Report on Zoophytes.* Beyond this line either side of the 

 equator we have no species of true Madrepora, Astrsea, 

 Meandrina, or Porites ; below this line these corals abound, 

 and form extensive reefs. This line is hence an important 

 starting-point in any map illustrating the geography of ma- 

 rine life. Passing beyond the regions of coral reefs, we leave 

 behind large numbers of Mollusca and Radiata, and the boun- 

 dary marks an abrupt transition in zoological geography. 



The next line below that of 68° F. is that of 74° F. The 

 corals of the Hawaiian Islands, and the Mollusca also to a 

 considerable extent, differ somewhat strikingly from those of 

 the Feejees. The species of Astraea and Meandrina are 

 fewer, and those of Porites and Pocillopora more abundant, 

 or at least constitute a much larger proportion of the reef 

 material. These genera of corals include the hardier species ; 

 for where they occur in the equatorial regions, they are found 

 to experience the greatest range in the condition of purity of 

 the waters, and also the longest exposures out of water. 



* In the author's Report on Geology, 66° F. is set down as the limiting tem- 

 perature of coral-reef seas ; this, however, is given as the extreme cold. 68° 

 appears to he the mean of the coldest month, and is therefore here used. 



