104 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
here a few examples illustrating the abundance of some of these 
common forms and some characteristics of their occurrence. 
The Diatoms often attain their greatest profusion succes- 
sively, not simultaneously, so that single genera, or it may 
even be single species of a genus, make up on occasions the 
bulk of the phytoplankton. For example, on May 29th, 1916, 
the haul consisted mainly of Chaetoceras, of which genus there 
were 19,461,600 individuals, and of these 19,396,000 belonged 
to the species C. sociale leaving only 228,900 as the number 
of all the other Diatoms present taken together. On such 
occasions, and this is only one example of many, a single species 
makes up nearly the whole of the plankton. The maximum 
on the Diatom curve ranges in different years from March to 
May. In 1907 it was in March, in 1908 in May, and in 1909 in 
April. In some years the Diatom maximum may be divided 
into two parts, an earlier due mainly to Chaetoceras and 
Thalassiosiva, and a later in June due to Riizosolena and 
Guinardia. A common order of succession for the species 
which contribute most largely to the Diatom maxima is— 
Biddulphia mobiliensis and Coscinodiscus (C. concinnus and 
C. radiatus) in early April, Chaetoceras debile in late April, 
Chaetoceras sociale in May, Chaetoceras teres and Rhizosolena 
shrubsoler in early June, and Rhizosolenra (several species) 
and Guinardia in later June. The autumn Diatom maximum 
is constituted mainly in the Irish Sea by Chaetoceras subizle 
and Rhizosolenia semispina. : 
A comparison of the monthly averages for the selected 
Diatoms throughout the years shows that Chaetoceras and 
Rhizosolema are by far the most abundant forms, and that 
sometimes the one and sometimes the other outnumbers its 
fellow in the proportion of nearly ten to one. But they do 
not occur in quantity together. Chaetoceras in March, April 
and May is succeeded by Rhizosolenia in May and June. A 
comparison of the largest hauls of Diatoms and of Copepoda 
in each year brings out how greatly the Diatoms outnumber 
