212 GEOGRAPHICAL DISTRIBUTION. 



genera Clypeaster, Echinanthus, Me tali a, Cidaris, Diadema, Echinometra, and 

 with the North Atlantic by the genera mentioned above, Echinocardium and 

 Strongylocentrotus. 



The fourth realm, the Australian, differs radically from the others, and re- 

 mains more completely isolated from the other existing realms. Though it 

 is associated by Centrostephanus and Brevnia with the Indo-Pacific, and by 

 Strongylocentrotus and Echinocardium with the North Atlantic Realm, the 

 South Circumpolar District must be considered a part of this realm (by Go- 

 niocidaris canaliculars), as w r e have looked upon the North Circumpolar Dis- 

 trict as a part of the North Atlantic Realm. 



The principal Isochrymal Lines dividing the oceans into Frigid, Temperate, 

 and Torrid are copied from Professor Dana's Isochrymal Lines, given by him 

 on his valuable map of the Geographical Distribution of Crustacea, in his 

 great work on the Crustacea of the United States Exploring Expedition. 



Plate A, contains the following Littoral Districts, — Pacific, Boreal American, West South 

 American, Tropical Atlantic, West Africa, Indo-Africa, Indo-Pacific, North Circum- 

 polar, Patagonia. 



Plate B, — Japanese, Australian, North Pacific, Panamic, North Atlantic, Lusitanian, South 

 Circumpolar, East Indian, California^. 



Plate G, shows the Distribution of the Oceans into four great Realms, — Atlantic Circum- 

 polar, Australian Antarctic, Pacific, and American. 



