PLANKTON OF THE GULF OF MAINE 435 



because the diatoms had then sunk from the surface (which was practically barren 

 of them) to a depth of about 20 meters, where they were congregated in such num- 

 bers that even the coarse-meshed net came back clogged. 



The same neritic species of Chsetoceras — laciniosum, contortum, debile, and 

 diadema — together with the more oceanic deci-piens, besides occasional Lauderia 

 glacialis, Thalassiosira gravida, and Coscinodiscus, were found over the coast banks 

 west of Nova Scotia in June, 1915 (there were few diatoms there in May; p. 3S7) and 

 dominated the much more abundant diatom plankton of that region in April, 1920 

 (stations 20103 to 21015). 



I may also note that in 1915 the easily recognized cells of Chsetoceras co'iv- 

 strictum, which I have not detected during the spring, appeared in some numbers 

 in the catches near Mount Desert Island on June 14 (station 10285) and in Petit 

 Passage, Nova Scotia, on the south side of the Bay of Fundy on the 10th. Appar- 

 ently this species reaches its plurimum in the gulf in mid-August, when we have 

 found it dominant off Mount Desert (station 10250, August 14, 1914). On the other 

 hand, the group of oceanic species that includes Ch. atlanticum, Ch. criophilum, and 

 Ch. densum (subgenus Pha?oceras of Gran, 1908), have as a rule been represented 

 sparsely in our siunmer hauls. They were cither wanting or at least very rare in 

 all our tow nettings during July and August of 1913 and 1915, although several of the 

 more neritic representatives of the genus (listed above, p. 418), with other diatoms, 

 occurred abundantly along the coast east of Penobscot Bay during those months. 

 In 1914 criophilum was an important though not the dominant element in the 

 diatom plankton off Penobscot Bay on August 14 (station 10250), and at the more 

 easterly stations the day before (stations 10247 and 10248), while Ch. atlanticum 

 and Ch. densum were likewise detected in these hauls. More data are needed to show 

 whether these oceanic species are to be expected regularly in the gulf in August 

 but have been overlooked in the cruises made during that month in other summers. 



If Chsetoceras plankton is characteristic of the western part of Georges Bank 

 in spring, as the abundance of Ch. sociale suggests, it must vanish before midsummer, 

 because no Cluetoceras were detected on the bank among the swarms of Guinardia 

 (p. 391) in July, 1913 or 1914; only an occasional Ch. densum and Ch. decipiens on 

 the southwest part of the bank on July 23, 1916 (station 10348) ; and no Chsetoceras 

 at all among the Thalassiothrix-Rhizosolenia community near by (station 10347, 

 p. 391). 



Chsetoceras decipiens is the only species of the genus that has been detected 

 consistently in the open gulf during the autumn months, and that only in the coast- 

 wise belt, 62 the only part of the gulf where diatoms of any kind occur in any number 

 at that season. Ch. debile, Ch. constrictum, and Ch. laciniosum have been recorded 

 locally along the coast of Maine in October (near Mount Desert Island, station 

 10328, October 9, 1915). C. danicum was also detected at this station — so far the 

 only record for this brackish-water species in the gulf outside the Bay of Fundy. 



The genus Chastoceras as a whole probably falls to its lowest ebb in the offshore 

 waters of the gulf late in December and early in January, at which season Ch. 

 decipiens and Ch. criophilum alone were detected at two stations (10488 and 10592) 



*» Stations 10310, 10310, 10317, 10318, 10332, 10323, 10327, and 10328, September and October, 191.1. 



