INTEODUOTION. 



The following observations were made while the writer was in charge 

 of a scientific expedition to the Bahamas under the direction of the Museum 

 of the Brooklyn Institute of Arts and Sciences. The report has received 

 the benefit of valuable suggestions from Professor Charles B. Davenport. 



The studies were made in the period extending from June 4-July 27, 1903, 

 and numerous surface hauls were made under most favorable conditions for 

 working. 



The medusa-fauna of the Bahamas is poor in comparison with that of the 

 Tortugas, Florida. For example, the writer secured ninety species of Hydro- 

 medusae, Scyphomedusae, Siphonophorse and Ctenophorse at the Tortugas, 

 while only forty-three species were found at the Bahamas. 



What is more interesting, the relative abundance of specific forms is 

 quite different in the two regions. Thus the three commonest species at the 

 Tortugas are Pseudoclytia pentata, Halitiara formosa, and Turritopsis nu- 

 tricula; yet the first was not found at the Bahamas, while the two others were 

 rare. On the other hand one of the commonest medusae of the Bahama 

 Banks, a new species of Lymnorea, has never been seen at the Tortugas, 

 and species of Netocertoides, Olindias, and Cubaia are abundant on the 

 borders of the Bahama Banks, but very rare at the Tortugas. 



Altogether, of 43 Bahama medusae, 23 are about equally abundant at 

 the Tortugas, and 13 of these are either Trachylina forms or Siphonophorae, 

 which are widely distributed over the Tropical Atlantic. 16 are more abun- 

 dant at the Bahamas than at the Tortugas, while 4 are more abundant at 

 the Tortugas. 5 medusae are known only from the Bahamas, and 2 of these 

 are locally abundant. 



These considerable differences in the medusa-fauna of two regions upon 

 the same latitude, and being only about 300 miles apart, are co-ordinated with 

 differences in local conditions. For example, the Tortugas lie to leeward of 

 the Gulf Stream, and the rich pelagic life of the great current is constantly 

 driven upon their shores; while the Bahamas lie to windward of the stream, 



