20 Geographical Distribution of. Crustacea. 
of which are common to the two. 'T'his showsa nearly equal dis- 
tribution between the zones. But excluding the Brachyura, the 
numbers become 501 to 811, giving a preponderance of more 
than one-half to the "Temperate zone.* 
The species of highest rank among the Brachyura, Macroums, 
Isopoda, and Amphipoda, the four principal types in the above; 
belong to the extra-torrid zones; and in subordinate groups or 
families, it is often true that the genera of superior grade are ex- 
tra-torrid, in contrast with.the others which are torrid genera. 
Higher groups, characteristic of the colder regions, sometimes 
show degradation among those species of the group that are trop- 
ical; and the tropical sections also may continue the line of 
degradation by an extension again into the colder seas. 
As we descend in the scale of Crustacea from the Podoph- 
thalmia to the "T'etradecapoda, the number of cold-water species 
increases, becoming in the latter group, three times greater 
than the warm-water species. [t is an important fact, neverthe- 
less, that this increase of cold-water species is still no mark of 
degradation ; the particular facts that have been discussed, lead- 
ing to a very different conclusion. Other principles follow. 
'l'hese are— 
F'irst, that the two types, the Decapodan and Tetradecapodan, 
are distinct types, to be independently considered, and not parts 
* Adding to the numbers above, the species which have been necessarily left out 
as of uncertain locality, amounting to one hundred and forty in all, and inserting also 
the Entomostraca, it makes the total of described living species in 1853, as follows :— 
Brachyura, : : , . . . 880 
Anomoura, . à : : ^ ; 262 
Macroura, : : A l ; NR 
—— —13889 
Anomobranchiata, AR SPORE RESP : 115 
Isopoda, : . 295 
Anisopoda, : : . : : v UMP 
Amphipoda, . ; : : : 941 - 
——— 693 
Entomostraca, . : s : : fuus 492 
Total, . . 2689 
The number of species collected in TM cruise of the Expedition (exclusive.of 
those lost in the wreck of the Peacock, which included nearly all the collections of. 
two seasons in the tropical regions of the Pacific) is nearly 900 ; and the number of 
new species described is 658, distributed among the groups as follows: 
Brachyura, : ; : ; : .UMBT 
Anomourà, . : ; : à : 50 
Macroura, s: ; : ; ; CE 
Anomobranchiata, H : : : E 28 
—— —286 
Isopoda, . : ; : : : . d 08 
Anisopoda, . ; E ^ ; ; Ü 
Amphipoda, : . . . . . MIU 
———184 
Entomostraca, : . . . : 188 
Total, : ; 1 . 658 
