THE MICROPLANKTON OF PLYMOUTH SOUND, 141 
THE DIATOMS. 
In estimating the diatoms, we find they fall naturally into two groups ; 
the first and most important includes the species beginning about April 
and usually ending about September, the second including those having 
their maximum in the winter or spring and extending from September 
2500 
200 Sa 
1500; 
1000 
900 
800 
700 
600 
500 
400 
300 
200; — 
| 
SEP, OCi.. NOV, "DEC, JAN, FEB: MAP APR, MAY JUNE) JUIEY  AUGS SEPT: 
1915 1916 
Fic. 2,—Curve showing average number of diatoms in 50 ce. for each month. 
10G 
or October to the end of March or April and May. In September these 
groups sometimes overlap, but the two large general maxima occur 
about April and from August to October, the diatoms of the first group 
thus being mainly responsible for both the spring and autumn maxima. 
The curve here given (Fig. 2) shows the average number of diatoms in 
50 ce. monthly throughout the year. The largest maximum is in April, 
although May comes very near. The autumn maximum here this year is 
