PLANKTON OF THE GULF OF MAINE 
421 
siosira may be taken as typical of this part of the coastwise belt during the first 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, Oh. diadema, Oh. 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 criophilum, 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, 
Tlialassiothrix 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 Chsetoceras 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 
8951—28 28 
