520 ABSTRACTS OF MEMOIRS. 



and it is probable that the zone of transition between the two Ceratium 

 forms was fairly constant in position on the southern half of the Banks 

 from April onwards. There is little to suggest that the transition repre- 

 sents a gradual change in process between two nearly related forms of 

 possibly the same species, prompted by changes in the physical con- 

 ditions. The observations lead to the view that the origin of each is 

 entirely distinct, that of C. arcticum being in the Labrador water on the 

 north, and that of C. longipes being in the south and west, a southerly 

 origin being also ascribed to the oceanic Diatoms that were found present 

 here as early as ilpril. 



As has been stated, the limitations of the neritic Diatoms as a whole 

 were found to conform closely with the hydrographic boundaries along 

 the outskirts of the Labrador Current, and only at those points where 

 the salinities and isotherms became irregular, as, especially, in the 

 region of the Flemish Cap, did the former become confused. This point 

 is brought out in the very low average temperatures which species show, 

 as compared with those obtained by the International Investigations in 

 the North European waters. For a large number of " abundant " 

 records, Thalassiosira Nordenskidldii, for example, shows an average 

 temperature of 0-9°, as against an International Investigations average 

 of 5-3° ; in T. gravida the average is 14°, as against 6-3° ; in C.sociale 

 it is 0-9°, against 3-9°. The sharpness of the limitations in distribution 

 usually occurring in this region seems to be due to the suddenness and 

 extent of the change which at most points occurs between the polar and 

 the Atlantic water. At the surface, wave movement was probably 

 accountable for the fact that frequently, when intersecting the boundary 

 of the polar water, a narrow intervening belt was traversed in which 

 both polar and oceanic Diatoms became almost or entirely absent, the 

 conditions being apparently intolerable to both. In the vertical direction, 

 at positions where different layers of water were superimposed, and 

 where little or no such mixing occurred, the vertical range of species was 

 found so sharply defined that it was measurable within a metre or less. 



I am indebted to the Council of the Marine Biological Association 

 and to Dr. Allen for having placed all facilities at my disposal at the 

 Plymouth Laboratory for the examination of the material and the 

 preparation of the report. 



L. E. C. 



