GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 283 



Caribbean Sea ; the number of species of Decapods in the former 

 region being only twelve, whilst in the latter it is seventy-one. 



II. The differences of form and organisation are not only more 

 numerous and more characteristic in the warm than in the cold 

 regions of the globe ; they are also more important. The number of 

 natural groups, which we find represented in the Polar and Tem- 

 perate Regions, is much smaller than that of which we find types 

 or examples in Tropical Seas. In fact, nearly all the principal 

 forms, which are met with in colder regions, also present them- 

 selves in warm ; but a very large proportion of the latter have 

 no representatives among the former. Thus, of the three primary 

 groups, into which the Class is at first subdivided, the Xypho- 

 sura are altogether wanting beyond the forty-fourth degree of 

 latitude. Again, the Brachyourous and Anamourous Decapods 

 appear to be altogether excluded from some of the most northern 

 regions that have been explored. Of the family of Squillidre 

 ( 791 ), so highly characteristic of the Order Stomapoda, it is 

 rare to meet with any members, north of the forty-fifth degree 

 of latitude. And the curious group of Phyllopoda is restricted 

 within a still nearer neighbourhood of the Tropical Region. 



III. Not only are those Crustacea, which are most elevated in 

 the scale, deficient in the Polar Regions ; but their relative num- 

 ber increases rapidly as we pass from the Pole towards the 

 Equator. Thus the Brachyoura, which must be considered as the 

 most elevated of the whole series, are totally absent, as we have 

 just seen, in some parts of the arctic region ; and we find their place 

 taken by the far less complete Edriophthalma, with a small num- 

 ber of Anomourous and Macrourous Decapods. In the Mediterra- 

 nean, however, the Decapods surpass the Edriophthalma in regard 

 to the number of species ; and the Brachyourous division predomi- 

 nates over the Macrourous, in the proportion of two to one. 

 And in the East and West Indies, the short-tailed are to the 

 long-tailed Decapods, as three, four, or even five, to one. Again, 

 the Land- Crabs, which are probably to be regarded as taking 

 the highest rank among the Brachyoura, are only to be met with 

 between the tropics. And of the fiuviatile Decapods (those 

 which inhabit rivers, brooks, and lakes), a large proportion 

 belong in tropical regions to the elevated type of the Brachy- 



