14 G'eoeraphical Distribution of Crustacea. 
IV. ANOMOBRANCHIATA. 
XXIV. The Mysidea, to which the Penzeidea are related, are, 
to a considerable extent, cold-water species, although many are 
found also in the tropies. . Tere are among them twenty torrid 
species and seventeen extra-torrid species. 
Iu the Squilloidea, we have an example of an inferior. grade in 
a large lax body, with a small head and long abdomen ; and they 
remind us of overgrown larval forms, or species vegetatively en- 
larged beyond the normal or most efficient size. In this particu- 
lar they have some analogies with the earlier forms of life. 
'l'hey are found mostly within the tropies. "T'wenty-four of the 
Squillidae are torrid zone species, and only seven pertain exclu- 
sively to the 'l'emperate zone. Of the Erichthide, twenty-one 
out of twenty-two species are reported from the 'l'orrid zone. 
The Amphionidea, a related group, include seventeen "'orrid 
zone species and two of the 'l'emperate zone. 
V. TETRADECAPODA. 
Before stating the conclusions from the tables* of the 'lT'etra- 
decapoda, it should be observed that this division of Crustacea 
has been less thoroughly explored than that of the Podophthal- 
mia, and future investigations must vary much the proportions 
between the species of the different regions. "l'he coasts of Eu- 
rope and the northern seas, are within the reach of European zo- 
ologists, and have been carefully examined ; while voyagers 
through the tropies have usually contented themselves with col- 
leeting 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. 'T'he ''etradecapoda are far more numerous in extra- -tropical 
latitudes than in the tropical. 
The proportion in the table is 521:146; allowing for future 
discoveries, it may be set down at 251 without fear of exceed- - 
ing the truth. 
IL. 'T'he genera of Bas aofical seas are far more numerous 
than those of the tropical. 
Out of the forty-nine genera of /sopoda, only nineteen are known 
to occur in the tropics, and but four of these are peculiar to the 
tropics. 
Out of twenty genera of An?sopoda, six only are keovnt to 
be tropical, and but two are exclusively so. 
* As already mentioned the Tables published in the original Report are here 
omitted. 
