BIGELOW: EXPLORATIONS OF THE COAST WATERS. 



327 



of diatoms, just as in the waters off northwestern Europe (Gran, 

 1915). When the diatom swarm is at its height in April, Ceratium 

 plays only a subordinate role in the plankton of the Gulf (1914b). 

 But by May the waters on the two sides of the Gulf were occupied by 

 Ceratium plankton (Fig. 98); and in June Ceratium had not only 

 replaced the diatoms right across the southern half of the Gulf, from 

 west to east, but also occupied the whole breadth of the continental 



Fig. 100. — Microplankton types, August-September 1915. 



3, Ceratium 



4, Mixed diatom and Ceratium 

 T, Occurrence of Thalassiosira 



shelf off southern Nova Scotia (Fig. 99), just as is the case in mid- 

 summer (p. 322). In September (Fig. 100) the entire Gulf, except 

 for small regions of mixed plankton off the coast of Maine and off 

 Massachusetts Bay, was occupied by Ceratium. 



The several species of Ceratium, like the prominence of the genus 

 as a whole, undergo a definite seasonal fluctuation in the Gulf. In 

 May, the predominant Ceratium at all the stations (except Stations 



