420 
BULLETIN OF THE BUREAU OF FISHERIES 
that Coscinodiscus was most numerous over the inner half of the shelf where it, 
together with the oceanic species Ghsetoceras criophilum and Gh. decipiens, composed 
the bulk of the catch (stations 20073 to 20075), but occurred only sparsely at the 
outer stations (stations 20076 and 20077) ; whereas neritic species (notably Ch. mitra, 
Ch. diadema, Gh. laciniosum, Gh. debile, and Thalassiothrix nitschioides ) were most 
plentiful over the outer half of the shelf (stations 10275 and 10276), not next the 
land as one might have expected, and even occurred outside the continental slope 
as well (station 20077). 
Such a concentration of neritic forms at the outer stations off Shelburne instead 
of at the inner is intelligible when hydrographic conditions are taken into account, 
because the axis of the cold Nova Scotian current of low salinity, itself essentially 
neritic in its biologic aspect, occupied precisely the same location at the time. 
The abundant diatom community already mentioned (p. 383) as characterizing 
the western part of Georges Bank on February 23, 1920, consisted chiefly of slimy 
masses of the tiny neritic species Ghsetoceras sociale, not of Coscinodiscus nor of the 
oceanic species of Chsetoceras, though Gh. decipiens, Gh. criophilum, and Gh. atlan- 
ticum all occurred there, as did the neritic forms Rhizosolenia shrubsolei, Eucampia 
zodiacus, and Leptocylindrus. This flowering of Gh. sociale was very local, as seems 
usually to be the case when concentrations of diatoms occur on Georges Bank, and 
was confined strictly to the comparatively shoal waters of the bank (stations 20046 
and 20047). Gh. sociale was sought in vain in the tow netting over the edge only 
20-odd miles distant (station 20045), where Thalassiothrix nitschioides and an 
occasional cell of Guinardia and Ghsetoceras diadema were the only neritic diatoms 
recognized. The very sparse community of diatoms in the basin immediately to 
the north of the bank (station 20048) consisted of the same oceanic species of diatoms 
that characterize the central parts of the gulf generally in February and March — that 
is, Coscinodiscus, Ghsetoceras atlanticum, Gh. criophilum, Gh. decipiens, Gh. boreale, 
Ch. densum, Rhizosolenia semispina, and Thalassiothrix longissima. 
No tropical phytoplankton was found at our stations outside the continental 
slope in February or March, 1920 (stations 20044, 20069, and 20077). 
Our work for 1913 had already suggested that the diatoms that first commence 
rapid multiplication in the Cape Ann-Cape Elizabeth region in spring are the fore- 
runners of the vernal flowerings that are the most spectacular event in the yearly 
planktonic cycle of the Gulf of Maine. These are the several species of Chsetoceras 
that may rival the peridinians here and there along the coast even as early as the last 
of January or early February, especially in Ipswich Bay. Shortly thereafter the 
genus Thalassiosira begins flowering, a phenomenon which we have been able to 
follow through parts of the years 1913, 1915, and 1920. 
In 1920 the tow at the mouth of Massachusetts Bay contained Thalassiosira, 
besides several other kinds of diatoms, on March 1 (station 20050; see list p. 423); 
and Thalassiosira and Chsetoceras must both have commenced flowering actively 
even earlier than this alongshore between Cape Ann and Cape Elizabeth that year, 
the “rich” diatom area outlined on the chart (fig. 104) being dominated by these 
two genera on March 4 and 5. 
The list given below (p. 425) for the station near Cape Elizabeth (20059), which 
was paralleled near the Isles of Shoals (station 20060), and the dominance by Thalas- 
