GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 



1455 



explained by the author in his Report on Zoophytes.* Beyond this 

 line either side of the equator, we have no species of true Madrepora, 

 Astrasa, 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 marine life. Passing 

 beyond the regions of coral reefs, we leave behind large numbers of 

 Mollusca and Radiata, and the boundary 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 pro- 

 portion 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. Their 

 abundance at the Hawaiian Islands, as at Oahu, is hence a conse- 

 quence of their hardier character, and not a mere region peculiarity 

 independent of temperature. There are grounds, therefore, for draw- 

 ing a line between the Hawaiian Islands and the Feejees ; and as the 

 temperature at the latter sinks to 74 1 ° F. some parts of the year, 74° 

 F. is taken as the limiting temperature. The Feejee seas are exceed- 

 ingly prolific and varied in tropical species. The corals grow in great 

 luxuriance, exceeding in extent and beauty anything elsewhere ob- 

 served by the writer in the tropics. The ocean between 74° F., north 

 of the equator, and 74° F. south, is therefore the proper tropical or 

 torrid region of zoological life. 



With respect to the line of 80° F., we are not satisfied that it is of 

 much importance as regards the distribution of species. The range 

 from the hottest waters of the ocean 88° to 74° F. is but fourteen 

 degrees, and there are probably few species occurring within the region 

 that demand a less range. Still, investigations hereafter made, may 

 show that the hot waters limited by the isocryme of 80° includes 

 some peculiar species. At Sydney Island and Fakaafo, within this 

 hot area, there appeared to be among corals a rather greater preva- 

 lence than usual of the genus Manopora, which as these are tender 



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

 of Coral-reef Seas : this, however, is given as the extreme cold. 68° appears to be the 

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



