GEOGRAPHICAL DISTRIBUTION OF 4 CRUSTACEA. 15H 



XVIII. It was found in the Brachyura, that the highest species 

 among the Maioids, and the highest of Crustacea occur in the extra- 

 tropical regions ; and that as we descend to the Cancroids, the species 

 become mainly tropical; moreover, as we descend among the Can- 

 croids (the type of which is tropical), there is in general a return to 

 the less genial colder waters, as exemplified in the true Cancers or 

 CancridaB and the Corystoidea, these last being mainly cold-water 

 species. By these steps we find the more degraded forms among the 

 Brachyura occurring in both the colder and warmer waters. "We 

 cannot therefore expect that the Anomoura, which are properly Bra- 

 chyura of a still lower grade, should be arranged according to rank in 

 one zone in preference to the other. And it is a fact that the genera 

 of higher species occur about equally in the two zones. Latreillia, but 

 a single step below the Inachidae, is found in the warmer temperate 

 regions; and Dromia, a little lower, has three-fourths of its species in 

 the tropics. Homola, again, has been found only in the temperate zone. 



Among the Paguridea, the Bernhardi or cold-water species are pro- 

 bably the superior in rank; and the Lithodea, which are a grade 

 higher still, are from the neighbourhood of the frigid zone. 



The Hippidea, which have been considered as in the Corystoid 

 series (page 54), but below the Corystoidea, are mostly from warmer 

 waters. 



The most bulky forms among the Anomoura are found in the 

 genera Lithodes, Ranina, and Dromia. The common Ranina dentata 

 has a length of five inches in the Japan Seas, while in the warm East 

 Indies (at the Moluccas), as De Haan states, four inches is the greatest 

 length. 



II. MACROURA. 



XIX. The Macroura, according to the table, are nearly equally 

 divided between the torrid and extra-torrid zones, the former including 

 one hundred and forty-seven species, and the latter one hundred and 

 fifty-three species. 



In the above table we have not included the fresh-water Astacidae, 

 as we are treating only of marine species. Yet in a comparison of 

 numbers between the zones, these should be brought in. They are 

 about thirty-six in number, and all, excepting perhaps one, belong to 



