1915] 



SETCHELL DISTRIBUTION OF MARINE ALGAE 



295 



shows a definite correspondence between certain of these 

 and the boundaries of different marine floral regions as previ 

 ously laid out and indicated in this paper. 



From the point of view of the distribution of the marine 

 benthos, so far as algae are concerned, it is found by practice 

 to be satisfactory to divide the surface waters of the ocean 

 into nine zones, as follows: Upper Boreal, Lower Boreal, 

 North Temperate, North Subtropical, Tropical, South Sub- 

 tropical, South Temperate, Lower Austral, and Upper Aus- 

 . The limiting isotherms of surface temperature chosen 

 those of the summer month or maxima, viz., the isotheres, 



tral 



which are those of February (or 



bly March) for the 



southern hemisphere and those of August 



bly Sep 



tember) for the northern hemisph 



These lines are laid 



down with approximate accuracy in the charts of the atlases 

 of the different oceans published by the ' ' Deutsche Seewarte ' ' 

 of Hamburg ( '92, '96, '02). These isotherms are more accurate 

 and explicit for the open ocean than for the neritic zone where 

 the algal benthos occurs, but, with certain allowances, the 

 as indicated are sufficiently accurate. 



Each of the zones I have proposed covers 5°C. range of 

 surface temperature with the exception of the Upper Boreal 

 and the Upper Austral, each of which includes a range of 



10°C 



■ » 



ghtly 



trarily adopted, are 



between the isotheres of 0°C 



r. The zones, then, more or less arbi 

 the Upper Boreal and Upper Austral 



even 



2°C 



to 10°C 



Lower Boreal and Lower Austral between the isotheres of 

 10° C. and 15° C, North Temperate and South Temperate 

 between the isotheres of 15° C. and 20°C., North Subtropical 



between the isotheres of 20° C. and 



and South Subtropical betwee 



25° C, and the Tropical between 25° C. and 30° C. (or above). 



These 5°C. zones are thus laid out according to the 5 

 isotheres, because on inspection these isotheres approach 



most closely or touch the shores at the drv 



points of 



floras and 



pal floral 



They have been de 



termined empirically, and indicate, as it seems from 



ence in working with them, that they coincide with floral 



boundaries the oceans over more exactly than do any of the 



