1528 CRUSTACEA. 



The species of highest rank among the Brachyura, Macroura, Iso- 

 poda, 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 extra-torrid, in con- 

 trast with the others which are torrid genera. Higher groups, cha- 

 racteristic of the colder regions, sometimes show degradation among 

 those species of the group that are tropical; 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 Podophthalmia 

 to the Tetradecapoda, the number of cold-water species increases, 

 becoming in the latter group, three times greater than the warm-water 

 species. It is an important fact, nevertheless, that this increase of 

 cold-water species is still no mark of degradation; the particular 

 facts that have been discussed, leading to a very different conclusion. 

 Other principles follow. These are — 



First, that the two types, the Decapodan and Tetradecapodan, are 

 distinct types, to be independently considered, and not parts of a 

 series or chain of species, a fact illustrated in the preceding chapter on 

 the classification of Crustacea. 



Second, that the preponderance of cold-water species is the reverse 

 of what must have been true in the earlier geological epochs, when 

 the oceans had a somewhat higher temperature ; or were to a large ex- 

 tent tropical. 



Third, that the progress of creation as regards Crustacea, has ended 



sive 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, . . . . .151 



Anomoura, ..... 50 



Macroura, . . . . .57 



Anomobranchiata, .... 28 



286 



Isopoda, . . . . . .67 



Anisopoda, ..... 7 



Amphipoda, ..... 110 



184 



Entomostraca, . . . . 188 



Total, . . 658 



