482 DISCOVERY REPORTS 



The thick line A represents the subtropical convergence. The entries to the left of 

 this line indicate which species extend beyond the areas under consideration, and it is 

 noticeable that the species with this wider distribution are a small minority. The details 

 of their distribution, which in some is extensive, are to be found in the systematic part 

 of this report. 



The thick line B between the Falkland Islands and Heard Island marks the Antarctic 

 convergence, all localities between lines A and B being between the two convergences 

 and hydrologically sub-Antarctic, those to the right of line B south of the Antarctic 

 convergence and hydrologically Antarctic. 



The definition of the Antarctic and sub-Antarctic areas by means of the Antarctic 

 and subtropical convergences gives them rather different limits from those accepted 

 before the detailed work of the 'Discovery' (Deacon, 1933, 1937) was available. 

 Hasenbank (1932, p. 323), for instance, regarded both Bouvet Island and Amsterdam 

 Island (Neu Amsterdam) as sub-Antarctic, but Bouvet Island is south of the sub- 

 Antarctic convergence and Amsterdam Island north of the subtropical convergence. 

 The Antarctic affinities of the Bouvet Island Polyzoa appear from Hasenbank 's own 

 remarks. From Amsterdam Island he had Scruparia ambigua (recorded as S. chelata) 

 which is known from the sub-Antarctic but is widely distributed in more northerly 

 waters; and Bugida dentata, a widely distributed tropical and subtropical species (see 

 p. 429). 



The division between the Antarctic and sub-Antarctic regions is the most con- 

 spicuous feature of Table 3, the great majority of the species being found in one region 

 or the other, but not in both. Only two of the few species common to the Antarctic and 

 sub-Antarctic are widely distributed in both areas. The variation of Caberea darwinii 

 in correlation with its distribution is described on p. 382. In Cornucopina pectogemma, 

 which is less widely distributed in the sub-Antarctic regions, variation is less marked, 

 and does not seem to be correlated with distribution. Among the remainder there are 

 three Antarctic species that are only found, when north of the convergence, in the deeper 

 water off the Patagonian Shelf (see p. 486), and four species known from South Indian 

 islands on both sides of the convergence (see p. 483), one of which {Bugula longissima) 

 reaches Ross Sea. 



The fact that varieties are treated independently in the table does not seriously 

 exaggerate the separation of the Antarctic and sub- Antarctic forms, for in only one 

 instance (Cornucopina ovalis) are the typical form and the variety restricted to opposite 

 sides of the convergence. 



As the Antarctic convergence is a phenomenon of the surface waters, it is, at first 

 sight, surprising that the distribution of bottom-living forms such as Polyzoa, should 

 be so closely correlated with it. It must, however, be remembered that the Antarctic 

 surface layer (Deacon, 1933, p. 173) is of considerable depth, and that the great majority 

 of the Polyzoa have been taken in relatively shallow water, and would be subject to 

 "surface" conditions. It has, moreover, been shown by Deacon (1937, pp. 3, 24) that 

 the position of the convergence at the surface is determined by conditions in the deeper 

 waters. His diagrams (1937, fig. 1, p. 4; fig. 5, p. 22) of the vertical circulation of the 



