208 GEOGRAPHICAL DISTRIBUTION. 



Echinarachnius parma on the east coast of North America strikes us as very 

 characteristic of a Canadian fauna, yet when we remember that it is found 

 on both sides of North America and on the east side of Asia, extending 

 probably far south into the great Indo-Pacific Littoral Realm, Ave at once see 

 that although we have on the east coast of North America several of these 

 littoral assemblages, we have in reality only two faunaa, one the Circnmpolar, 

 the other the West Indian, which meet at Cape Cod, and by combination 

 with the Pacific Boreal, the Lusitanian, the Atlantic, and the Mediterranean, 

 form littoral assemblages which have thus far received the name of fauna?, 

 but are evidently not entitled to it. 



The Littoral Districts, which have been mapped out on PL A and B, are 

 not made from any preconceived notions, but have been taken from the dis- 

 tribution of the several species of the annexed lists. We cannot fail to be 

 struck with the remarkable coincidence of the great belts of temperature 

 first mapped out by Dana, which are here copied from his map on the geo- 

 graphical distribution of Crustacea, ami the approximate limits of many of 

 our littoral districts, — an agreement which is still more striking when we come 

 to examine the range of the principal genera. 



The number of littoral districts recognized is considerably smaller than the 

 number of provinces adopted by Forbes. Woodward, and Dana, the principal 

 writers on the limits of marine geographical provinces. This is due in part 

 to the smaller number of species of Echini, but the principal cause of the 

 extensive range of so many species must be looked for in the influence of 

 the currents. 



The effect which currents play in shaping the geographical distribution of 

 marine animals is very great; we have an example in the Gulf Stream and 

 the northern branch of the Amazonian current flowing into the Gulf of 

 Mexico, which account fully for the great range of the more common littoral 

 species. The Japanese current makes itself fell as far as San Diego, two 

 species of Echini extending in the Northern Pacific from the northern part 

 of Japan along Kamtchatka. the Aleutian Islands. Sitka. Vancouver's Island, 

 the one as far as Cape Mendocino ( S. Drobachiensis), the other (Echinarach- 

 nius excentricus) to San Diego. The Indo-Pacific equatorial current has 

 undoubtedly been the main agent of the extensive geographical range of 

 such species as Cidaris metularia, Echinoneus cyclostomus, Heterocentrotus 

 mammillatus, Diadema setosum, Ilipponoe variegata. Echinolampas ovifor- 

 mis, Brissus carinatus, Clypeaster humilis. 



