Geographical Distribution of Crustacea. 7 
ologists, and have been carefully examined; while voyagers 
through the tropics have usually contented themselves with col- 
lecting the larger Crustacea. In the genus Gammarus, not a 
tropical species had been reported, until our investigations, which 
rought 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. 4 
he proportion in the table is 521: 146; allowing. for future 
discoveries, it may be set down at 2:1, without fear of exceed- 
ing the truth. 
Il. The genera of extra-tropical seas are far more numerous 
than those of the tropical. 
Out of the forty-nine genera of Isopéda, 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. : é 
_ The variety of extra-tropical forms compared with the tropical, 
is hence very great. 
IIT. 
Idoteidea, is . 8:1 
: eee | 
thoideay.° 2g: 1 
D Gammaridea are most strongly extra- 
ion being for the extra-tropical and tropical 
