PLANKTON OF THE GULF OF MAINE 
443 
(;■ ition 20109), and in 1913 it was prominent in the rich diatom flora over the north 
it part of the banks during the last few days of the month, as noted above (p. 422) 
May, 1915, it was not uncommon among the more plentiful Chsetoceras and Tha* 
dosira in the deeps of the gulf (stations 10267 to 10269) and was dominant locally 
;re on the 10th (station 10273) and near the Isles of Shoals on the 14th (station 
!78). It was also recorded in Ipswich Bay on the 8th in 1920 (station 20122), 
t it was not detected at all on the western part of Georges Bank and neighboring 
asin, in the Massachusetts Bay region, in the coastal belt north and east of Cape 
Elizabeth, nor off western Nova Scotia during that month, either in 1915 or 1920. 
Rh. semispina was not found among the abundant diatom flora of the Mount 
sert region in June, 1915 (e. g., station 10285), or in the offshore parts of the gulf 
a ring that month, but there was a scattering of it among the Thalassiosira and 
setoceras in Petit Passage on the 10th, and it might fairly be classed as domi- 
lt over German Bank on the 19th (station 10290). 
Our midsummer records for this species are confined to Georges Bank (where 
:c asional cells were noted in July, 1914, stations 10219 and 10223, but none at all 
among the Rh. styliformis, Rh. shrubsolei, and Thalassiothrix longissima that swarmed 
July 23, 1916); to the Eastern Channel (station 10227), Browns Bank (station 
28), the neighborhood of Lurcher Shoal (station 10245), the northeast corner of 
tne gulf (stations 10247 and 10248), the waters off the coast of Maine east of Cape 
Elizabeth (station 10258) ; and to the shelf off Marthas Vineyard, where it swarmed 
on August 25, 1914 (station 10258; fig. 125). Like diatoms generally, Rh. semispina 
practically vanishes from the central deeps of the gulf during the summer. Nor is 
there any reason to look for a considerable augmentation in its numbers there during 
the autumn, for it has appeared only sparingly in our September, October, and 
November hauls (station 10047, November 20, 1912; stations 10317 and 10336, 
September 15 and October 26, 1915; and stations 10400 and 10403, November 1 
and November 8, 1916). It was widely distributed over the northern half of the 
gulf (always, however, in very small numbers) in the midwinter of 1920-21, when it 
occurred at about 50 per cent of the stations (stations 10490, 10491, 10494, 10495, 
10496, 10497, 10500, and 10502). Fritz (1921a) records a scattering of “Rh. hebe- 
tata,” which probably were this variety, at St. Andrews in every month except 
November. 
The most notable feature of the occurrence of Rh. semispina in the Gulf of Maine, 
as outlined by our data, is its irregularity; no definite succession of flowerings is 
demonstrated. On the whole, however, it can be described as at its maximum 
during the spring and summer (this half of the year includes all the rich flowerings 
we have encountered), and at its minimum in autumn and winter. At Woods Hole, 
too, Fish (1925) reports the richest flowerings of this species as occurring in summer. 
This parallels its seasonal status in northern European seas, where it is most abun- 
dant from April until June, flowering earliest in the more southern and latest in more 
northern waters. 64 But no definite correlation between flowering periods and latitude 
or temperature is yet apparent for the Gulf of Maine. 
Flowers most abundantly in the North Sea in May, but not until August in Greenland waters and in Barents Sea. 
