178 PROF. W. A. HERDMAN ON THE DISTRIBUTION 
There are certain differenees in detail. For example, the total Diatom 
curve at Plymouth has three maxima or crests, in April, August, and October. 
At Port Erin the curve has only two erests, a much greater maximum in 
spring and a variable and smaller one in autumn, while Diatoms are usually 
wholly absent in August. 
On the other hand, there is a general agreement in regard to the distribution 
throughout the year of many of the more abundant organisms. For example, 
amongst Diatoms Coscinodiscus is a winter and early spring form, Biddulphia 
flourishes throughout the winter from November to April or May, Rhizosolenia 
is a summer form having its maximum in June, while Chetoceras and 
Lauderia have two maxima, the one in spring and the other in autumn, in the 
English Channel and the Irish Sea alike. Amongst Copepoda there seems 
to be a general agreement along with a certain amount of difference in detail 
which will be referred to below when discussing the species. 
I may recall that in November 1910 I read a paper before this Society * 
in which I made a comparison between the summer (July) plankton on the 
West Coast of Scotland and that of the Irish Sea, showing that in some of 
the deep fjord-like highland sea-lochs green-coloured phytoplankton can be 
obtained even in the height of summer, while a zooplankton may be found 
living in abundance a few miles away. This, of course, would be impossible 
in the Irish Sea, where a zooplankton and a phytoplankton do not occur 
simultaneously. 
DIATOMS. 
The seven generic forms I have selected for consideration taken together 
make up nearly the whole of the Diatom plankton of the year. No other 
genus occurs in anything like such profusion as these. In April, for example, 
when the Diatoms are usually at their climax, all the remaining genera 
(at most 10 or 12) taken together make up only about one-thousandth, or less, 
of the whole. Moreover, these common Diatoms often attain their greatest 
profusion successively, not. simultaneously, so that single genera, or it may 
be single species of a genus, make up on occasions the bulk of the phyto- 
plankton. For example, in May 1916 the month's average haul of Diatoms 
was 7,171,789, while the average for the genus Chwtoceras taken alone was 
6,947,333, leaving only 224,456 as the average of all the rest of the Diatoms. 
On the last two individual hauls, taken on May 25th and 29th, the actual 
numbers were as follows :— 
Chetoceras alone... ie 24,094,500 de 19,461,600 
C. sociale alone bis A 23,936,000 5s 19,396,000 
All other Diatoms together... 166,300 ne 228,900 
So that on these occasions, and they are examples of many, one species makes 
up nearly the whole of the plankton. 
* Journ, Linn, Soc., Zool. xxxii. (1913) p. 23. 
