OF DIATOMS AND COPEPODA IN THE IRISH SEA. 107 



records have occurred during these same mouths, in 1910, 1911, 1912, 1913, 

 1914, 1915, and 1916. 



The highest monthly averages for Thalassiosira are 721,000 in April 

 1912, 898,000 in May 1913, and 976,000 in May 1915. The genus is 

 totally absent from our records throughout the ten years in December, 



^■4^5^,4 -I'- •-•-•>,-'?■.■ "A? -"V f 



Fig. 8. — Photo-micrograph of a phytoplankton consisting mainly 

 of Thalassiosira Nordenskioldi. 



January, and February, and again in July and August, and is very poorly 

 represented in several other months. In fact, it is only really abundant in 

 April and Ma}' and not always then. In this distribution over the months 

 of the decade this genus forms a marked contrast with C/nrtoceras, which is 

 so constantly present. 



Rhizosolenia. 



The species that occur most abundantly in our records are Rhizosolenia 

 semispina, R. Shrubsolei, R. Stolterfothi and, less abundantly, R. setigera and 

 R. alata (rarely). The two last named are temperate Atlantic forms, and 

 R. semispina is a typically arctic oceanic species. 



They are all summer or autumn species, the genus being often quite 

 unrepresented in certain of the winter months. It usually begins to appear 

 in the nets in February and reaches its maximum in June. The numbers 

 are sometimes enormous, giving the water in a glass jar a characteristic silky 

 or fibrous appearance and causing the June crest that is usually present as a 

 distinct elevation on the Diatom curve (see fig. 5. p. 182). 



The most abundant species in the Irish Sea is Rhizosolenia Shrubsolei — at 

 Plymouth it is R. Stolterfothi • but R. semispina (fig. 9) is sometimes present 

 in abundance at Port Erin in autumn, causing a second crest or maximum in 

 September or October. In September 1907, thirteen millions and sixteen 

 millions of this species were taken in two hauls of the surface-net at Station 

 III. (three miles off Port Erin), while the following year, at the same time 

 and locality, it was almost absent, 



