GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 1497 



We here notice a few of the general facts or conclusions that may 

 be deduced from the preceding tables. 



I. The line of division, separating the Torrid and Temperate zones 

 of ocean temperature, following the isocryme of 68 or the outer limit 

 of coral reef seas, marks a grand boundary in organic life, well exem- 

 plified in Crustacean species. Out of the five hundred and thirty-four 

 species of the Torrid and Subtorrid Regions (the Torrid zone), there 

 are one hundred now known to be common to the two. But of the 

 two hundred and fifty-four in the Temperate Regions, only thirty-four 

 occur in the Torrid zone. A large number of genera, containing more 

 than a single known species, are confined wholly to the Torrid zone : 



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