PLANKTON OF THE GULF OF MAINE 421 



siosiramay be taken as typical of this part of the coastwise belt during the firsl half 

 of March. A few miles farther out at sea, however, on the same day, between 

 the Isles of Shoals and Jeffrey's Ledge (station 20061), the several species of Chseto- 

 ceras, combined, dominated instead of Thalassiosira, though there was also a con- 

 siderable amount of the latter in the catch; in fact, practically a repetition of the 

 list of species given for station 20059 (p. 425). 



In the spring of 1921, when we found the vernal flowering just commencing along 

 the western shores of the gulf during the first week of March, there was a typical 

 though still only moderately plentiful Thalassiosira-Chsetoceras plankton in Massa- 

 chusetts Bay on the 4th (station 10505), dominated by the former, with Chsetoceras 

 debile, Ch. didymum, Ch. diadema, Ch. decipiens, Biddulphia aurita, Ditylium bright- 

 wellii, Coscinosira, Coscinodiscus, Lauderia borealis, and Rhizosolenia semispina. 

 Thalassiosira nordenskioldi, with Biddulphia aurita, also dominated a very sparse 

 diatom plankton in Ipswich Bay that same day (station 10506), with a strong sprink- 

 ling of Ditylium brightwellii, a few Chsetoceras criophilvm, Lauderia, and Coscino- 

 discus. North of this (stations 10507 and 10508) and farther offshore (stations 

 10509 and 10510) the water was still almost clear of diatoms except for Coscinodiscus. 



In a tow near Seguin Island, March 4, 1920 (station 20058) Lauderia glacialis, 

 not Thalassiosira or Chsetoceras, dominated a moderately plentiful diatom plankton, 

 which also included Chsetoceras decipiens, Ch. debile, Ch. diadema, and other species 

 not yet determined, Rhizosolenia semispina and R. setigera, Thalassiosira nordenskioldi, 

 Thalassiothrix nitschioides, and Coscinodiscus. The assemblage of species was much 

 the same near Mount Desert Island the day before, though the plankton was extremely 

 scanty (station 20056; see list, p. 426). The inference from this is that Lauderia 

 began flowering in this zone earlier in the season than either Thalassiosira or Chseto- 

 ceras. We have found no evidence of such a sequence either between Cape Cod 

 and Cape Elizabeth in the one side of the gulf or off western and southern Nova 

 Scotia in the other (the latter marked "sparse diatom" on the chart, fig. 104), 

 where tows during the second and third weeks of March, 1920, shortly antedating 

 the local flowerings of Thalassiosira and Chsetoceras, yielded no Lauderia at all but 

 were dominated by Coscinodiscus, the diatom flora, as a whole, still being very 

 sparse, though including a considerable list of species (see list, p. 427; stations 20072, 

 20078, and 20084). 



In the coastal waters of the gulf the genera Thalassiosira and Chajtoceras are 

 the most characteristic members of the diatom flora of spring; it is unusual for any 

 other to dominate there after the vernal flowerings are well underway. 



Rapid multiplication of Thalassiosira and Chsetoceras is responsible for the 

 expansion of the extent of rich diatom plankton which takes place in the western 

 side of the gulf from March on (p. 385). In 1920 Thalassiosira nordenskioldi, Chsetoceras 

 debile, and C. decipiens together dominated the plankton in Massachusetts Bay 

 on April 6 (stations 20089 and 20090) , with a considerable list of other species less 

 numerous (see list, p. 424). 



The swarms of diatoms off Cape Ann (station 20091), northward past Cape 

 Elizabeth, across the mouth of Casco Bay, and seaward out to Platts Bank (stations 

 20091 to 20096) also consisted chiefly of Thalassiosira and of various species of 



