182 
PROF. W. 4. 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. 
in the Irish Sea than in the English Channel, judging from the Plymouth 
records. 
Both Biddulphia and Coscinodiscus seem to be slightly later in their maxima 
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 elose 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 
CH. ETOCERAS. 
The chief species of this genus in our Irish Sea plankton are Chetoceras 
boreale, C. contortum, C. crioplilum, C. debile, C. decipiens (fig. 6), C. densum, 
April 
8 millions 
Oheetoce y= 
Rhizosole ma 
| million 
1 Chaetoceras 
1 \ Sept: 
V M + 
" \ M Rh ixosoleni a 
—; Nm atu duci iir ioia io 
Jan. Feb. Mar. Apr. May June July 
] 
Aug. Sept. Oct. Nov. Dec. 
Fia. 5.— Curves of the more important Diatoms of ihe April and June maxima. 
C. diadema, C. sociale, and C. teres. Of these, C. boreale, C. criophilum, and 
C. decipiens are Arctic oceanic forms, C. densum is a temperate occanic or 
