162 DISTRIBUTION. 



jS'ortli America as another province of hydroid distriljution distinct from that of the northern, 

 north-eastern, and north-western shores of the Athmtic. 



Again, the hjdroid fauna of the Mediterranean has many special forms, and differs so much 

 from those of the other two regions, that we are compelled to raise it also to the rank of an 

 independent province. 



Three definite provinces of hydroid distribution may thus be regarded as well established 

 in the great North Atlantic area. These may be designated as the Boreo-Celtic, Northern 

 Atlanto-American, and Mediterranean. 



The West Indies will probably constitute a fourth, the hydroids of the West Indian seas being, 

 so far as w^e know, very distinct ; the species, however, with which we are acquainted from this 

 locality are not yet sufficiently numerous to justify us in regarding a West Indian province as 

 fully established. 



The data we have from other parts of the world are also very imperfect. Among these the 

 Pacific shores of North America, New Zealand, Australia, and South Africa are the regions from 

 which we have obtained the most abundant information ; and though we are as yet far from 

 knowing as much of their hydroid faunas as could be desired, I believe we are justified in regarding 

 all but South Africa, whose jjresent hydroid fauna would appear to be a spurious one, originating 

 largely in artificial immigration — as so many distinct provinces of hydroid distribution. In the 

 North Pacific Ocean especially, we would seem to have a singularly well-marked area, for we have 

 seen that, with the exception of Serfularia jmuiila and Lafoiia dumosa — species which may have 

 been easily carried on the bottoms of ships — not a single species inhabiting this region has yet 

 been proved to occur in any other part of the world- 



The exact limits of this North Pacific Province cannot yet be assigned. It would seem, 

 judging from the species which I find common to the two, to extend at least from the North of 

 Vancouver's Island to the South of California. We know nothing of the hydroids of the Asiatic 

 shores of the Pacific, but, arguing from the greater continuity of the coast line by wiiich the 

 North Pacific Ocean is encircled, we may well believe that there is a greater agreement in their 

 hydroid faunas between the two sides of the North Pacific than between the two sides of the 

 North Atlantic. 



Besides the three Provinces enumerated above as determined with sufficient certainty in the 

 North Atlantic, four others may thus with good reason be indicated as probable, namely, the 

 West Indian, the North Pacific, the Australian, and the New Zealand. 



The generalisations now given are the utmost that we are justified in founding on the facts 

 before us. Wherever in other parts of the world hydroid trophosomes have been met with, it is 

 only one or two species that have been collected or recorded from any one region — an amount of 

 material quite inadequate for the indication of definite provinces. 



h. Veriicnl Dutiihidion. 



General covsidera1ions.—1\\Q vertical or bathymetric distribution of the IIydroida has, like 

 their horizontal distribution, been very imperfectly investigated, and beyond the North Atlantic 

 area we have scarcely any available data. 



Until quite recently we should have deemed it sufficient to express the vertical distribution 



