18 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 21 



Faunal Relationships 



Separated from Australasia by the Central Pacific oceanic barrier, 

 the Pacific coast of North and South America is the most isolated con- 

 tinental coastline in the world. Nevertheless, where it approaches the 

 Asiatic mainland very closely at its northern terminus, some faunal inter- 

 change occurs, both via the Aleutian Island chain and at Bering Strait. 

 Several species of boreal Pacific Majidae, among them Oregonia gracilis, 

 Hyas coarctatus, and Chionoecetes opilio, are common to American and 

 Asiatic coasts, while the last two named range northward and eastward 

 through Beaufort Sea and Davis Strait to the North Atlantic. 



At the opposite terminus of this continuous coastline, in the Cape 

 Horn region, the so-called "antiboreal" species communicate with the 

 South Atlantic via the Strait of Magellan, ranging northward to south- 

 ern Brazil. Among these may be mentioned Eurypodius latreillei, ( ?) 

 Taliepus marginatus, Leurocyclus tuberculosus, and Libidoclaea grana- 

 ria. There is no American majid common to Antarctica (cf. Pfefler, 

 1890) , but Paramithrax baeckstroemi of Juan Fernandez Island is closely 

 related to P. peroni of New Zealand and was earlier referred to that 

 species (Lenz, 1902). 



On the other hand, certain tropical Pacific species, among them 

 Acanthonyx petiveri, Notolopas lamellatus, Inachoides laevis, and 

 Microphrys weddelli, are known to occur in, or have been attributed to, 

 the tropical Atlantic as well. The Pacific and Atlantic portions of their 

 ranges are no longer continuous and, following the evolutionary pattern 

 of the vast majority of majid species, the respective populations are already 

 on their way to becoming distinct, albeit homologous, species. 



It will be seen, therefore, that the boreal Pacific American fauna 

 relates itself most closely to the Asiatic Pacific and circum-Arctic, 

 whereas the austral Pacific American fauna is continuous with the South 

 American Atlantic. The tropical Pacific American fauna relates itself 

 to the tropical Atlantic American by means of cognate or geminate 

 species, and, by means of its genera, to the tropical faunas of the world. 





