HYDROMEDUSAE, SIPHONOPHORES, AND CTENOPHORES. 351 



Out of the total of 48 Philippine-Malayan species no less than 21 

 are definitely known from some part of the western half of the 

 tropical Pacific, chiefly from Polynesia ; that is to say, about as large 

 a proportion as is common to Malayan and Philippine waters. And 

 the agreement is the more striking in view of the fact that nearly 

 all our modern knowledge of the medusa-fauna of the central tropical 

 Pacific is based on the three collections made by Agassiz on his ex- 

 peditions to Fiji (Agassiz and Mayer, 1899), through Polynesia 

 (Agassiz and Mayer, 1902) and to the eastern Pacific (Bigelow, 

 1909a). It is evident, then, that a very uniform medusa-fauna ex- 

 tends from the Malay region eastward at least as far as the Paumotos. 

 But while about 17 of the Malaysian species are known from the 

 western coast of America, most of these, as Aglaura, the two species 

 of Liriope, Geryonia, Bhopalonema, Solmundetta, Aegina, Probo- 

 scydactyla, and Cytaeis, are typically oceanic, or " holoplanktonic," 

 forms; either without hydroid stage, or placed in the oceanic class, 

 so far as dispersal is concerned, by their asexual budding. Of 

 the remaining species which are common to Malaysia and West 

 America, three, Bougainvillea fulva, Eutima levuka, and Leuckar- 

 tiara octona, extend from one side of the Indo-Pacific to the other; 

 the latter being practically cosmopolitan in the Atlantic as well ; and 

 Philalidium discoida and Slabberia brownei i are probably also dis- 

 tributed over the entire breadth of the Pacific. In short, very few 

 leptoline forms, and those few very widespread, are common to the 

 two sides of the tropical Pacific, which supports the view, already 

 advanced by me (1909a) that the broad uninterrupted oceanic area of 

 the eastern half of the tropical Pacific has been an effective barrier 

 to dispersal of such forms. Conversely, there is a considerable fauna 

 peculiar to the west coast of America, only a small proportion of 

 which extends westward to the islands of the central Pacific, or to its 

 western part; while, on the other hand, there is an unmistakable 

 resemblance between the leptoline forms of the west coast of America 

 and of the tropical Atlantic, to which I have called attention else- 

 where (1909a). 



The collections from Hongkong and from Japan, recently re- 

 corded by Maas (1909), by Kishinouye (1910), by Vanhoffen (1912), 

 and by me (1913), give some slight but welcome data on the northern 

 extension of the tropical species of the central part of the Indo- 

 Pacific area. 



The small list from Hongkong (Vanhoffen, 1912) is typically 

 tropical ; that is Euphysora bigeloivi, Irenopsis hexanemalw, Phortis 

 palkensis, Eutima levuJca, Aequorea pensile, Aglaura hemistoma. 

 Liriope tetraphylla and Solmundella bitentaculata. But only 11 of 



1 Not included in table because not known from the Philippines or Malaysia (1909a). 



