BIGELOW: EXPLORATIONS OF THE COAST WATERS. 



325 



numbers of the spirally coiled Chaetoceras debile intermingled with the 

 long rod-like Thalossiothn'x longissima, and an occasional Coscino- 

 discus. But both Thalassiosira and Chaetoceras debile were lacking 

 in the central part of the Gulf, being replaced there by other species of 

 Chaetoceras, especially C. dcnsum and C. horeale, while the diatom 

 component of the mixed plankton in the eastern side of the Gulf was 

 chiefly Rhizosolenia semispina and Chaetoceras. By the middle of 



'•^•a 



Fig. 98. — Microplankton types. May 1915. 

 5, Diatom 



3, Ceratium 



4, Mixed diatom and Ceratium 

 , ofiFshore limit to Thalassiosira 



June (Fig. 99) pure diatom plankton had dwindled to a small area off 

 Mt. Desert (Station 10285), i. e. was no more extensive than in the 

 preceding August (p. 322, Fig. 97). Diatoms had now practically 

 disappeared from the southwestern and central parts of the Gulf 

 (Stations 10298, 10299), where they had constituted the bulk of the 

 plankton a month earlier, while Thalassiosira gravida, T. norden- 



