352 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



the Malay-Philippine species are known to reach Japan, and all of 

 these are widely distributed throughout the warm waters of the Pa- 

 cific and Indian oceans ; nine of them are also known in the Atlantic. 

 That is to say, none of the characteristic tropical leptoline forms 

 (1913, p. 109) penetrate to Japan, though the oceanic Medusae are 

 carried thither by the Kuro Shiro current. 



As I have pointed out (1913, p. 109), the probable explanation for 

 the absence of the tropical leptoline species from Japan is that they, 

 or their hydroids, can not survive the cooling of the water in winter ; 

 and this working hypothesis is supported by the fact that the tropical 

 Gonionemus suvaensis is replaced there by G. vertens var. depressum; 

 a variety of the species found on the temperate west coast of America, 

 and closely allied to the one known from the corresponding zone on 

 the coast of New England. The subtropical genus Olindias, too, is 

 replaced in Japanese waters by Olindioides, and Rathkea octcmemalis 

 by R. hlumenbachii. 



Rhopalonema, Aglaura, and Liriopc, and the other holoplanktonic 

 medusae of warm waters are, on the contrary, limited in their ex- 

 treme dispersal by the summer, not by the winter temperature. And 

 though Japan is within the range of Rhopalonema, Aglaura, and 

 Liriope in summer, it is doubtful whether they would be found there 

 in winter. 



In tracing the Malay-Philippine species westward we are met 

 by the difficulty that while the combined data from the Maldives 

 (Bigelow, 1904, Browne, 1904) and from Ceylon (Browne. 1905), 

 (which can no longer be looked on as having separate medusa - 

 faunae), and from the Chagos Archipelago (Brown, 1916), give a 

 preliminary survey of the central portion of the Indian Ocean, our 

 knowledge of the medusae of its western side is very scanty. 



About 19 Malayan species are so far known from the Maldives or 

 Ceylon, while of the 23 hydromedusae recorded by Browne (1916), 

 from the Chagos Islands, 16 at least have already been recorded from 

 Malaysia. And several other species from the central part of the 

 Indian Ocean, as for example, Slabbei^ia b? 7 ownei, Gonionemus su- 

 vaensis, and Turritopsis nutricula, probably occur in the Philippine 

 region as well, since they have been taken in the tropical Pacific, or 

 in Japanese waters. In short, there is no evident separation between 

 Malaysia and the Ceylon-Maldive-Chagos region, so far as their me- 

 dusae are concerned. 



Since a continuous coast line, with tropical sea temperatures, con- 

 nects the two sides of the Indian Ocean, there would be no reason 

 to expect to find their medusa-faunae different. And so far as they 

 go, the few records available suggest uniformity. Thus Proboscydac- 



