108 



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, 17.'! millions on June 3rd, 59 millions on June 8th, 43 millions on 

 June 13th, all in 1912, and 17 millions on June 21st, 1915. 



Fig. 9. — Photo-micrograph of a phytoplankton consisting mainly 

 of JRhizosolenia 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 

 (fio'. 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 



