16 bulletin: museum of COMrAKATIVE ZOOLOGY. 



keeper of the lighthouse at Loggerhead Key, Tortngas, to whose in- 

 telligent and painstaking co-operation we owe much that may be of 

 value in the following paper. We also wish to thank Major J. E. 

 Sawyer, U. S. A., to whose permission we were indebted for the use of 

 tlie government steamer, " George W. Childs," for transportation to and 

 from between Key West and the Tortngas. We are also indebted for a 

 like service to the officers of the Union Bridge Company in allowing 

 the use of their steamer " Ambrosio Bolivar." 



The Tortugas occupy what is probably the most favorable situation 

 from which to study the pelagic life of the Tropical Atlantic. They lie 

 upon the northern edge of the deep channel of the Gulf Streani as it 

 issues from the Gulf of Mexico. Pure, deep ocean water surrounds 

 them, and there are none of the shallow mud-flats that render the shore 

 waters of Florida so turbuleut, at times, that many of the more delicate 

 pelagic animals are killed. As is well known, the Gulf Stream pours 

 outward from the Gulf of Mexico through the Straits of Florida. The 

 Gulf Stream does not occupy the whole cross-section of the strait, how- 

 ever, but according to the researches of Lieutenant, now Commander, 

 J. E. Pillsbury (Report U. S. Coast and Geodetic Survey, 1885-87), 

 it flows nearer to the Cuban coast than to the line of the Florida Keys. 

 The northern limit of this great stream lies at least 28 miles south of 

 Rebecca Shoal, the average edge being about 6 miles farther south, or 

 34 miles south of Rebecca Shoal (see U. S, Coast Survey Report, 1887, 

 pp. 174, 175, Illustration 42). 



The currents in the immediate vicinity of the Tortugas are extremely 

 variable and are greatly under the influence of the tides and winds, 

 while the tides themselves are small and easily influenced by extraneous 

 circumstances. In the passage between Rebecca Shoal and the Tortu- 

 gas the current sets practically north with the flood tide and south with 

 the ebb. About five miles west of Loggerhead Key the southerly set 

 of the ebb tide is stronger than the northerly current induced by the 

 flood. There can be no doubt that the prevailing winds play an im- 

 portant part in setting up local currents in the immediate vicinity of the 

 Tortugas. The prevailing E.-S.E. winds of the summer months cause a 

 decided westerly surface drift, and this is evidenced by the fact that dur- 

 ing this period sand is washed away from the eastern shore of Logger- 

 head Key and spread out into long cuspate forelands ^ which extend from 



» " Cuspjite foreland " is a term used by F. P. Gulliver (1896 ; Bull. Gcol. Soc. 

 America, Vol. VII.) to denote a sandy, projcctinij point of land which has cuspate 

 outlines, and is formed by the agency of currents. 



