434 BULLETIN OF THE BUEEATJ 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 Chsetoceras. 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, Ch. 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 Chastoceras species from April until June. For example, the list for a 

 station (10278) north of Cape Ann for May 14, 1915, includes Ch. densum,, Ch. 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 Cha3toceras 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. 



Chsetoceras 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, 



