386 
BULLETIN OF THE BUREAU OF FISHERIES 
diatoms swarming and peridinians practically nonexistent during the first half of 
May in 1915 (Bigelow, 1917, p. 324), the result being that the vernal flowerings 
of diatoms reach their widest expansion at this season. 
Fig. 105. — Distribution of the more characteristic types of phytoplankton, April, 1920. 1, rich diatom; 2, sparse diatom; 
3, sparse Ceratium and diatom; 4, rich Phaeoeystis and diatom 
The unicellular alga Phseocystis may also swarm, even to the extent of monopo- 
lizing the surface waters locallj 7 , for a brief period during the month of April, but 
shortly disappear once more, as occurred in the southern part of Massachusetts 
