vol. l.] Torrey. — Hydroida of th< Pacific Coast. 7 



which 7 arc local)-, three of the tour species of Haleciidai are 

 local: the Clavidat and Hydractiniidat arewithoul representatives. 



The differences in the fauna which have just been outlined are 

 correlated with certain geographical differences. The Japan 

 Current, striking the shore of Alaska, parts into two streams. 

 The larger turns down alone' the coast of the United States; the 

 smaller turns upward and runs along- the southeast shore of the 

 Alaska Peninsula. The shore of this part of the Alaska coast 

 drops precipitously away, so that the current comes close in toward 

 the land. North of the peninsula is the great one hundred fathom 

 plateau, extending many hundreds of miles out from the shore 

 line. On this plateau the hydroids from Northern Alaska have 

 been collected. It is covered bj waters much colder than those 

 south of the peninsula. If the warmer waters of the Japan < 'urrent 

 reach it. they flow, so far as known, only along its edge. 



North of the peninsula, then, is a region whose waters are 

 largely covered with ice for more than half of each year. South 

 of the peninsula bee-ins a vast stretch of coast which is washed bj 

 the comparatively warm waters of the Japan ('urrent. This cur- 

 rent is probably accountable for the absence of abrupt transitions 

 between the faunal areas which I have tried to schematize above, 

 and the exceedingly lone- distances to which some of the northern 

 species have been distributed southward. The temperature of 

 the current varies gradually with the latitude, however, and that 

 offers some explanation for the small faunal differences that exist. 



Future exploration will doubtless reduce these differences. 

 While the Alaska coast from Unalaska to Sitka has been rather 

 thoroughly explored, practically no collections have been made 

 between Sitka and Vancouver Island. From Puget Sound to San 

 Pedro little is known of the off-shore fauna, though much dredg- 

 ing and shore collecting have been done in Puget Sound and 

 San Francisco and Monterey bays. Little is known of the 

 hydroid fauna on each side of Point Conception, a natural barrier 

 which has decidedly affected the distribution of some groups of 

 animals, notably the molluscs. 



