446 BULLETIN OF THE BUREAU OF FISHERIES 



it is high (over 35 per mille, as Ostenfeld suggests) and that water less saline, say, 

 than 33 per mille operates as an actual bar to its dispersal and propagation. Other- 

 wise it would be hard to explain its failure more completely to colonize the Gulf of 

 Maine, which is fully as accessible to it, both by temperature, by the influx of offshore 

 water, and by its geographic location, as the northern part of the North Sea is, where 

 Rh. styliformis occurs in abundance throughout the half year from May to November. 



Inasmuch as Rh. styliformis occurs chiefly as an immigrant in the Gulf of Maine, 

 where its presence is indicative of ocean water, it is one of the diatoms for which 

 a sharp lookout should be kept, a lookout facilitated by its large size and precise 

 structural characters. 



Rhizosolenia setigera is the antithesis of Rh. styliformis in its relation to the coast 

 line, for it is neritic instead of oceanic and produces resting spores, corresponding to 

 which difference it occurs more regularly in the Gulf of Maine. Its period of greatest 

 abundance falls in spring. Its richest flowerings roughly correspond with those of 

 the abundant Thalassiosira-Chsetoceras flora in their geographic locations, having 

 been limited in 1920 to the Cape Ann-Cape Elizabeth belt (stations 20058, 20059, 

 and 20061) and to one locality off Yarmouth (station 20083) in March, spreading to 

 Massachusetts Bay on the one side of the gulf (stations 20089, 20116, and 20117) 

 and to the banks off Nova Scotia, to Browns Bank, and to the northeast corner of the 

 gulf on the other (stations 20098 and 20099) by the last week of April. McMurrich 

 (1917), too, found this species attaining its maximum abundance in the St. Andrews 

 region in April, though Fritz (1921) does not list it at all from that locality. At 

 Woods Hole, however, Fish (1925) found rich flowerings in late summer as well as 

 during the winter and early in spring. 



Rh. setigera either diminishes in numbers in the open girif during May and June 

 or has been overlooked there among the more numerous diatoms of other genera, 

 for we have only one definite record of it for each of these months (station 10277 

 on May 13, 1915, and station 10299 on June 26, 1915). But it occurs occasionally 

 throughout the summer and at least until early October in coastal areas wherever 

 diatoms persist so late in the season in any quantity; off Penobscot Bay and in the 

 Mount Desert region, for example; near Machias, Me.; and on German Bank (stations 

 10029 and 10030 in 1912; 1024S and 10250 in 1914; and 10301, 10305, 10317, and 

 10318 in 1915). In the Bay of Fundy this species apparently passes through a period 

 of abundance in September and October (Bailey, 1917), an interesting phenomenon 

 paralleling its occurrence on the other side of the Atlantic, where it has two max- 

 ima — one in spring and the other in autumn (Ostenfeld, 1913). We have found 

 nothing to suggest this in other parts of the Gulf of Maine, however, or in Massa- 

 chusetts Bay. 



Rh. setigera was recognized at only two stations during the December to 

 January cruise of 1920-1921 (stations 10490 and 10502), and not at all in Massa- 

 chusetts Bay during the winter of 1912-13. Rh. setigera has not been found on 

 Georges Bank, on Browns Bank, in the Eastern Channel, or over the continental 

 slope to the south. The chart (fig. 128) illustrates the sharp contrast between the 

 distribution of the neritic species, Rh. setigera, and that of its oceanic relative, Rh. 

 styliformis. 



