MAYER: MEDUSAE FROM THE TORTUGAS, FLORIDA. 21 



found in this region consisted almost entirely of forms of Trachylina, 

 composed of Trachynemida), Aglauridae, and especially Geryonida?. 

 (See Craspedoten Medusen der Plankton Expedition, 1893, Taf. 

 VII., VIII.) 



The facts then appear to be that we have at the eastern extremity of our 

 " warm zone," or in that region adjacent to the coast of Africa and in 

 the neighborhood of the Canary Islands, a Hydromedusan fauna com- 

 posed of both Trachylina and Leptolina, and the species which compose 

 this fauna show a distinct relationship with Mediterranean forms. In 

 the midst of the " warm zone," midway between the Canary Islands 

 and the West Indies, the fauna is composed almost entirely of forms of 

 Trachylina that are pelagic species par excellence, and are distributed 

 widely over the high seas, and also reach the coasts of Africa and 

 America. In the Bahama-Tortugas region we find a Hydromedusan 

 fauna composed of both Trachylina and Leptolina, the Leptolina 

 forms of which are almost wholly distinct from those of the Canary 

 Islands. 



We wish to call attention to the fact that a comparison of the Hydro- 

 medusan fauna of the Bahama-Tortugas with that of the Canary Islands 

 is open to serious objections, and that the conclusions arrived at through 

 such a comparison may be of but little value. The Canary Islands 

 occupy a small area, and are surrounded by water of 1000-2000 

 fathoms in depth, while the temperature of the surface water in their 

 neighborhood is about 10° F. colder than that of the Bahama-Tortugas 

 region. We might then expect that a marked difference would be 

 observed in the Hydromedusan faunae of the two regions, for in the 

 neighborhood of the Bahamas and Tortugas we find great areas of very 

 shallow water having a very high temperature, while even the deepest 

 parts of the Gulf of Mexico and Caribbean Sea have a temperature of 

 39^° F. It would be much fairer and far more conclusive, were we able 

 to do so, to institute a comparison between the fauna of the Bahama- 

 Tortugas and that of the Gulf of Guinea in the neighborhood of the 

 Islands of Anno Bom, St. Thomas, and Fernando Po ; for here the con- 

 tinental slope of the African coast is more gradual than at any other 

 place, and the islands are surrounded by a depth of water not greater 

 than 500 fathoms, having a bottom temperature of 39^° F. ; which 

 is exactly the same as that of the deep parts of the Gulf of Mexico. 

 The temperature of the surface water is also nearly the same as that of 

 the Bahama-Tortugas region. The conditions at the Tortugas in 

 August and September are very similar to those in the Gulf of Guinea 



