INVESTIGATION OF THE PLANKTON OF THE IRISH SEA, 153 
year, even in January and February, but do not attain to their maxima 
(millions per haul) until March, April, or even May. Other species, such as 
boreale and sociale, are later in appearing (April or May), and then usually 
reach the millions very rapidly. A few species (boreale, debile, decipiens, 
densum, and teres) may on occasions re-appear in September or even later in 
autumn, and under favourable circumstances run up rapidly to a secondary 
maximum. 
The months in which the Chetoceras records are most numerous and reach 
the highest numbers per haul are March, April, May, and September, and of 
these the one with the greatest number of high records (in most cases 
millions) is May, the next being April, and then September. 
The most barren months of the year, not merely for Chetoceras and 
Rhizosolenia but for records of diatoms in general, are November, December, 
January, February, and August. July would also be very poor were it not 
that the June maximum of Rhizosolenia (see below) on occasions extends into 
the beginning of July. 
If we trace the prevalence of the five most abundant species of Chetoceras 
throughout the years, we find :— 
C. boreale has its maximum in May, and again in Sept. 
C. debile " » April, May, " Sept. 
C. decipiens ,, " March, April, ^ Sept. 
C. sociale ,, 0» May. 
C. teres ^» » April, May, » Sept.--Oct. 
Rhizosolenia. 
Out of the half-dozen species of Rhizosolenia that occur in the Irish Sea, 
there are only three that appear in our plankton gatherings in such abundance 
as to be of real importance, viz., À. semispina, R. Shrubsolii, and Ft. Stolter- 
fothi, ind of these the last-named is rarely (1913) as abundant as the other 
two. The months in which these three species oecur are May, June, July, 
and occasionally again in September ; but confining attention to the later 
spring or early summer phytoplankton maximum, which is clearly due to 
Rhizosolenia, the range of the three species mentioned is from the latter part 
of May through June and in some years into the early part of July. June 
is, then, the central month of this Æhizosolenia maximum, and June plankton 
gatherings are generally characterised by the dark-brown silky deposit 
which indicates the presence of /thizosolenia in quantity. 
Individual years may differ in being earlier or later, and also in the species 
which is present in greatest abundance—for example, 1908 and 1911 and 
'12 had the Ahizosolenia maximum early, in May and June; 1913 and '14 
later, in June and July; while in 1919, ’20, and ’21 the Rhizosolenia 
maximum extended over parts of all three months. Moreover, the three 
