1502 CRUSTACEA. 



their highest perfection in these coldest waters, determine the prin- 

 ciple as regards themselves, the highest grade of Crustacea. Lower 

 grades occur also in the colder waters, and the laws governing their 

 distribution demand separate study and consideration. 



IX. Passing a step below the Maioids, we come to the Cancroids 

 and these, with the exception of the lower Corystoid species, and 

 only one-eighth of the rest, are Torrid zone species. 



X. If the Torrid zone is the proper region for the full development 

 of the Cancroid type, and its heat is needed for this end, it is natural 

 that species of Cancroids like the Bortunince, Platyonychidce, and Can- 

 cridce, found in the less genial waters of the Temperate zone, should 

 bear some mark of inferiority, and it is a fact that they have such 

 marks in their structure. This inferiority is not seen in their smaller 

 size, for a larger size, under certain conditions, may equally evince a 

 lower grade, but in the inferior concentration of the life-system, 

 exhibited either in the lax outer maxillipeds, the elongation of the 

 antennae and abdomen, or in the smaller size or swimming character 

 of the posterior legs. 



For a like reason also, the species of Corystoidea, a grade still lower, 

 naturally occur in the cold and ungenial region they frequent. 



We hence perceive, that the degradation among the Maioids takes 

 place when the species become warm-water species, and the degrada- 

 tion among the Cancroids, in the reverse manner, when the species 

 become cold-water species ; for the reason that the colder waters are 

 the proper habitat for the Maioid type, and the warmer for the Can- 

 croid type. 



XI. In the tables of the Maioidea and Cancroidea of the Temperate 

 and Torrid zones, page 1499, the species are included by families 

 and subfamilies, and consequently the peculiarities of some genera are 

 not shown. In the families or subfamilies referred to the cold-water 

 section, there is only one warm-water genus, viz., Doclea, of the sub- 

 family Libi7iince, in which there are four Torrid and one Temperate 

 zone species. 



Among those referred to the warm-water section, there are the 

 following cold-water genera : — 



Species in Species in 



Torrid zone. Temperate zone. 



Parthenopinea, genus Eurjnome, ... 2 



(t u Eurjnolambrus, 1 



Xanthidae, " Paraxanthus, . . .0 2 



Ozinas, " Ozius, ... 2 3 



