OF DIATOMS AND COPEPODA IN THE IRISH SEA. 185 
records have occurred during these same months, 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, 
Fre. 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 May and not always then. In this distribution over the months 
of the deeade this genus forms a marked contrast with Chetoceras, which is 
so constantly present. 
RHIZOSOLENTA. 
The species that occur most abundantly in our records are Rhizosolenia 
semispina, R. Shrubsolei, Ft. 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 Phizosolenia Shrubsolei—at 
Plymouth it is 7. 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, 
