PLANKTON OF THE GULF OF MAINE 
457 
our other towings on the bank irrespective of season, is best explained as due to a 
drift of the Nova Scotian current moving southwestward in spring from the Scotian 
banks across Browns Bank and the eastern channel and along the outer part of Georges 
Bank. This is corroborated by sundry other lines of evidence, planktonic as well 
as hydrographic. 
As there is some confusion between this species and the closely related Th. 
frauenfeldi in the European lists published by the International Committee for the 
Exploration of the Sea (Ostenfeld, 1913), I may note that only such cells as were 
attached to one another in their characteristic zigzag chains are recorded here as 
nitschioides, these being quite different in appearance from the chains of frauenfeldi. 
The latter species has not been identified in any of the Gulf of Maine tow nettings. 
Other diatoms 
The genera so far discussed include all that we have found important in the 
plankton of the outer waters of the Gulf of Maine, and while the station lists (p. 423) 
include various others, none of them occur regularly or abundantly enough to color 
the plankton. I may emphasize especially the universal rarity of brackish-water, 
littoral, and bottom-dwelling diatoms out at sea. Pleurosigma, for example, is never 
represented by more than occasional examples, though detected at many localities 
far and wide. Under estuarine conditions, however, as in the tributaries of the Bay 
of Fundy, littoral diatoms of many genera are much more abundant (Bailey, 1917; 
Fritz, 1921; Bailey and Mackay, 1921). 
Finally, I may emphasize our failure to find any diatoms in the gulf to which 
it is safe to ascribe either a Tropic or an Arctic origin, except, perhaps, for Fragilaria 
oceanica, occasional examples of which were detected in the tows in the Eastern 
Channel and over the southeast slope of Georges Bank on April 16, 1920 (stations 
20107 and 20109). The absence of other arctic diatoms in the Gulf of Maine is 
the more striking if contrasted with their abundance and frequent dominance in 
the Gulf of St. Lawrence in spring, as is illustrated by the following table based on 
Gran’s (1919) list for May 11, 1915. This Arctic community proved so shortlived 
there, however, that it had entirely disappeared in June, to be replaced by a typically 
boreal assemblage, most of whose members — Rhizosolenia setigera, Nitschia seriata, 
Coscinodiscus, and Chsetoceras laciniosum — are equally characteristic of the spring 
plankton of the Gulf of Maine. 
St. Lawrence diatoms, May 11, 1915 
Arctic 1 
Gulf of 
Maine 
St Lawrence diatoms, May 11, 1915 
Arctic 1 
Gulf of 
Maine 
x 
Fragilaria cycliDdrus 
x 
Amphiprora hyperborea 
X 
Fragilaria oceanica.. 
x 
x 
X 
Navi cula pelagica 
X 
X 
Navicula septentrionalis 
x 
x 
Navicula vanhoffeni 
X 
x 
Nitschia closterium 
X 
Chsetoceras criophilum 
X 
Nitschia frigida 
X 
Chaetoceras debile 
X 
Pleurosigma stuxbergi 
x 
X 
Rhizosolenia hebetata 
X 
X 
Thalassiosira bioculata 
X 
Thalassiosira gravida 
x 
X 
X 
Thalassiosira hyalina 
X 
x 
Detonula confervacea 
X 
Thalassiosira nordenskioldi 
x 
X 
x 
1 Species that are endemic in the Polar seas, where ice forms in winter, and in the Gulf of St. Lawrence, but which occur only 
as immigrants farther south. 
