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186 PROF. W. A. HERDMAN ON THE DISTRIBUTION 
We have had much larger numbers, of R. Shrubsolei, in June; and amongst 
our largest records of the genus taken as a whole are :—184 millions on 
May 30th, 173 millions on June 3rd, 59 millions on June 8th, 43 millions on 
June 13th, all in 1912, and 17 millions on June 21st, 1915. 
Fra. 9.—Photo-mierograph of a phytoplankton consisting mainly 
of Rhizosolenia semispina. 
The highest monthly average is in June in all of our years except 1907 when 
it was in May, and 1913 and 1916 when it was in July. The greatest average 
recorded is over 40 millions in June 1912. 
The autumn records are very irregular and sometimes show no rise 
whatever. Even when present it is slight compared with the June maximum 
(see fig. 5, p. 182). The highest monthly average in autumn is 117,122, in 
September 1912. 
GUINARDIA. 
Only the one species, Guinardia flaccida, occurs in our records 
(tig. 10). It is a summer form occurring mainly between April and July, 
with the maximum almost invariably in June. Our largest records are 
22,800,000 on June 3rd, 1912; 18,000,000 on May 30th, 1912; and 
8,773,000 on June 11th, 1910. We have several records of over 7,000,000 
early in June, one of over two millions on July 8th, 1913, and one of over 
a million as late as July 16th, 1916. 
Guinardia, when present, makes up along with Rhizosolenia the June 
elevation, which is sometimes so marked towards the end of the vernal 
Diatom maximum. This last summer (1917) it was unusually late, showing 
a maximum of five millions on 23rd July, along with one million of 
Rhizosolenia. 
The highest monthly averages are all in June with the exception of 1916, 
when it was in July. The greatest recorded average is close on six millions 
for June 1912, The lesser, secondary, increase in autumn has its highest 
