SECT. 4] 



BIOLOGICAL SPECIES, WATER-MASSES AND CURRENTS 



401 



these connections since the Cretaceous. All antarctic and subantarctic species 

 of euphausiids are confined to the Antarctic Ocean and are circumpolar. 



'20° 160° 160° 120° 80' 



120° 160° 160° 120° 80° 40 



120° 160° 160° 120° 80° 40 



120° 160* '60° 120° 80° 40° 0° 40° 80° 



120° I 6C 160° 120° 80° 40° 



160° 160° 120° 80° 40° 0° 40° 80° 



Fig. 9. (a) Present range of the euphausiid Thysanoessa gregaria compared with hypo- 

 thetical circumstances of oceanwide warming (b) and cooling (c), both by 2|°C. The 

 7°C and 11°C isotherms at 200 m depth, associated with present limits of distribution 

 of T. gregaria, are considered limiting for the purpose of the extrapolation. With 

 warming (b), the 7°C isotherm (at the high-latitude boundary) moves to position of 

 present 4£°C isotherm, and the 11°C isotherm (at the low-latitude boundary) moves to 

 position of present 8£°C isotherm. With cooling (c) the limiting 7°C and 11°C iso- 

 therms move to positions of present 9£°C and 13£°C isotherms, respectively. 

 14 — s. ii 



