GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 



usually contented themselves with collecting the larger Crustacea. 

 In the genus Gammarus, not a tropical species had been reported, 

 until our investigations, which brought ten or eleven to light, being 

 one-third the whole number of those of ascertained localities reported 

 to this genus. 



Some general conclusions may, however, be safely drawn from the 

 facts already known, although the exact ratios deduced from the 

 tables may hereafter be much modified. 



I. The Tetradecapoda are far more numerous in extra-tropical 

 latitudes than in the tropical. 



The proportion in the above table is 521 : 146 ; allowing for future 

 discoveries, it may be set down at 2 : 1, without fear of exceeding the 

 truth. 



II. The genera of extra-tropical seas are far more numerous than 

 those of the tropical. 



Out of forty-nine genera of Isopoda, only nineteen are known to 

 occur in the tropics, and but four of these are peculiar to the tropics. 



Out of twenty genera of Anisopoda, six only are known to be 

 tropical, and but two are exclusively so. 



Among the Amphipoda, out of fifty genera of Gammaridea, only 

 seventeen are known to contain tropical species; nine are exclusively 

 tropical, and but ten, including these nine, have more tropical than 

 extra-tropical species. The Caprellidea and Hyperidea embrace thirty 

 genera, fifteen or sixteen of which include tropical species. 



The variety of extra-tropical forms compared with the tropical, is 

 hence very great. 



III. From the tables, the ratio of extra-tropical and tropical species 

 in the 



Isopoda, is . . . . .4:1 



Anisopoda, . . . . . 6:1 



Amphipoda, . . . . . .3:1 



Among the Isopoda, the Idotseidea are the most decidedly cold-water 

 species, and the Cymothoidea, the least so. The ratio of species for 

 the 



Idotasidea, is . . . . .8:1 



Oniscoidea, . . 7:1 



Cymothoidea, . . . 2J : 1 



Two-ninths of the extra-tropical Idotasidea (or nine species) belong 



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