104 



PROF. W. A. HERDMAN ON THE DISTRIBUTION 



autumn. The October and November monthly averages never, in our series 

 of years, come anywhere near the averages for March and April. 



Both Biddulphia and Coscinodiscus seem to be slightly later in their maxima 

 in the Irish Sea than in the English Channel, judging from the Plymouth 

 records. 



In the years 1907 to 1912, inclusive, the highest monthly averages (March 

 and April) for Coscinodiscus run in general about 100,000. In 1913 and 

 1916 they are close on 200,000, in 1914 over 300,000, and in 1915 there is a 

 sudden jump, in April, to over 840,000. The other months of that year do not 

 show unusually high numbers. 



CH2ETOCERAS. 



The chief species of this genus in our Irish Sea plankton are Chwtoceras 

 horeale, C. contortum, C. criopliilum, C. debile, C. decipiens (fig. (>), C. densum, 



8 -million* 



, r i Jun>. 



Jan. Feb. Mar. Apr. May June July Aug-. Sept. Oct. Nov. Deo. 

 Fig. 5. — Curves of the more important Diatoms of the April and June maxima. 



C. diadema, C. sociale, and C. teres. Of these, C. boreale, C. criopliilum, and 

 C. decipiens are Arctic oceanic forms, C. densum is a temperate oceanic or 



