BIOGEOGRAPHICAL BOUNDARIES 207 



or Thalassiothrix longissima) . Others were abundant over the 

 shelf but showed a sudden decline in numbers at the position of 

 the 100-fathom contour; for example, Calaniis finmarchicus which 

 followed the line of the continental slope around the British Isles 

 but also reflected the position of the relatively shallow waters 

 of the Wyville-Thompson Ridge between Scotland and Faroe 

 and northwestward across to the Icelandic shelf. The change in 

 the abundance and species composition of the plankton in the 

 region of the 100-fathom contour is so marked that it is frequently 

 possible to detect the position of the contour by a visual inspection 

 of the Recorder silks. 



As was to be expected, the distribution of a number of organisms 

 appeared to reflect the inflow of oceanic and coastal water from 

 the Atlantic into the northern North Sea. For example, Thalas- 

 siothrix longissima; this, and similar species, were found in the 

 Atlantic and, extending by a tongue, into the North Sea. Other 

 species, with their main centers of distribution in the southern 

 North Sea, appeared to retreat from this tongue of Atlantic in- 

 fluence (see Centropages hamatus, Fig. 1). 



No attempt has been made, in this atlas, to describe the dis- 

 tributions in the conventional terms of biogeography. The dif- 

 ferent organisms may be fitted more easily into a continuous series 

 than into separate types of distribution. In Fig. 2 (from Colebrook 

 et al.) the plankton has been listed in such a series, the position 

 of each organism being dependent on the location and extent of 

 its abundance and distribution in the Recorder Survey. Each 

 species was distributed in a similar way to its immediate neigh- 

 bors in the list. 



Although the series is continuous, it is possible to divide it into 

 a number of loosely defined parts. A large group of organisms 

 tended toward an oceanic distribution; a smaller group was largely, 

 but not exclusively, neritic; and a third group, showing distribu- 

 tions between and overlapping these extremes, can be described 

 for convenience as intermediate. Moving along the series one moves 

 progressively toward or away from oceanic through intermediate 

 to neritic distributions. All these terms are relevant only to the 

 Recorder Survey; they were selected as being useful and descrip- 



