434 
BULLETIN OF THE BUREAU OF FISHERIES 
the oceanic species criophilum, densum, and atlanticum were more generally repre- 
sented there than at the inshore stations, others as distinctively neritic — e. g., 
diadema and debile — were equally universal, and the latter was dominant at three 
out of these six stations (stations 20102, 20103, and 20106). 
In the offshore deeps of the gulf, where Thalassiosira never dominates the 
plankton, the augmentation of diatoms characteristic of late spring is chiefly due to 
these same species of Chaetoceras. Thus the April lists for these waters (stations 
20097, 20098, 20112, 20113, 20114, 20115, and 20116) are much the same as those 
for March (p. 418), Ch. criophilum, Gh. atlanticum, and Ch. decipiens being practically 
universal even in the most oceanic parts of the gulf, with Ch. diadema, Ch. lacini- 
osum, Ch. contortum, Ch. didymum, and Ch. debile less regular though widely dis- 
tributed. The latter, in spite of its neritic affinities, dominated a very rich assem- 
blage of diatoms in the Eastern Channel (p. 429, station 20107) and was abundant 
off the southern face of Georges Bank (station 20109) on April 16, 1920, although 
Ch. decipiens, Ch. atlanticum, Ch. criophilum, and Ch. densum were the only species 
of Chaetoceras noted on the shallows of the bank itself at that time (stations 20110 
and 20111). The fact that Ch. densum, which was apparently confined to Georges 
Bank during March, 1920, had spread to the southeast part of the basin by mid- 
April (stations 20112 and 20113), foreshadows the great abundance to which it 
attains in May (p. 429). 
On the assumption that the status of the various diatoms was essentially the 
same in the gulf in 1913 and 1915 as in 1920, little change takes place in the general 
association of Chaetoceras species from April until June. For example, the list for a 
station (10278) north of Cape Ann for May 14, 1915, includes Ch. densum, Gh. deci- 
piens, Ch. laciniosum, and Ch. debile, with Ch. contortum and Ch. didymum nearby 
(station 10277). Even where Ch. debile is the only species of Chaetoceras mingled 
in any abundance with the swarms of Thalassiosira, as is sometimes the case in April 
and May when Thalassiosira may practically monopolize the plankton, various 
other species of Chaetoceras can usually be detected by sufficient search. 
The vernal augmentation of Ch. densum just mentioned resulted in such an abun- 
dance of this diatom by the first week of May, 1915, that it either dominated the 
plankton or at least played that role jointly with Ch. criophilum over the western, 
central, and eastern deeps of the gulf generally (stations 10267 to 10269). In fact, 
these two, -with smaller amounts of Ch. decipiens, were almost the sole components 
of the rich diatom plankton (fig. 121) at the first-named locality (station 10267); 
but few if any Ch. densum had reached the northeast corner of the gulf (station 
10273) by that time, nor have we ever found this oceanic species an important factor 
in the phytoplankton near the land, where Ch. decipiens, Ch. diadema, Ch. contortum, 
Ch. debile, and Ch. didymum have proved the most plentiful representatives of 
their genus during May. 
Chaetoceras sociale in great abundance dominated the phytoplankton on the 
western part of Georges Bank in the last week of February (station 20046) and again 
on May 17 (station 20128) in 1920, suggesting that it continued flowering actively 
there throughout this period of more than two months. But apparently its season 
of reproduction was drawing to a close on our second visit to that general locality, 
