154 SIR W. A. HERDMAN : RESULTS OF CONTINUOUS 
species succeed one another in the order—semispina, Shrubsolii, Stolterfothi. 
The May records are mostly of semispina, which does not occur so frequently 
in June and hardly ever in July. KR. Shrubsolii is rare in the May 
gatherings, forms the greater part of those in June, and may also be 
abundant in early July. A. Stolterfothi only appears at the end of the 
Rhizosolenia maximum, late in June and early in July. In 1913 it was 
recorded in millions in July. 
The following statement gives the distribution of these more prominent 
species through the different years, and shows that—as in the case of 
Chatoceras 
is appropriate to speak of 1912 as a “shrubsolii” year, 1920 as a “ semi- 
spina” year, and so on. 
a species may be characteristic of a particular year, so that it 
1907—Poor year, no millions, only 2, semispina reached 3 mill. (Sept.). 
1908— Better; semispina and Shrubsolii both just reach millions (May and June). 
1909—Poor year, only semispina abundant (June). 
1910-—Better ; semispina (May); Shrubsol every haul in June (=44 mill. in all, average 
over 6 mill.) and early July. mE 
1911—Good; semispina abundant (late May and early June). 
1912—A * Shrubsolii " year; both semispina and Shrubsolii reach millions late May and early” 
June, but Shrubsolii the most abundant (about 100 mills. in several hauls), average 
for June over 40 mills. 
1913—Shows succession: semispina early, then Shrubsolii abundant (June), then Stolterfothi 
(mills. July) after SArubsolit. 
1914— Only Shrubsolü (June and July). 
1915—A nother “ Shrubsolii year (June and early July), aver. for June— over 10 mills, 
1916— Poor year, only SArubsoli reached mill, once (July). 
1917-—Same as 1916, 
1918—A “semispina” year, millions in May ; Shrubsolit few, 
1919--A * Shrubsolii ” year, appeared unusually early and reached millions late in May, 
remained through June and into July. 
1920—A “semispina” year, over 60 mills. late in May (average for May about 17 mills.), 
extends through June and part of July. 
A “Shrubsolii” year, begins late May, maximum middle June (over 8 mills.) ; 
Stolterfotht abundant end of June ; semispina almost absent. 
1921 
The only other genus of diatoms that sometimes approaches Chetoceras 
and /èhizosolenia in numerical importance is Thalassiosira, of which two 
“species occur in our Irish Sea plankton— 7. gravida and T. Nordenskioldii, 
and the latter is the more abundant. It may be regarded as a neritic 
northern species, and its occurrence in quantity in our seas can be taken as 
an indication that arctice water and northern plankton have invaded the 
British area. In April 1907 there was apparently such an invasion, and that 
spring Thalassiosira Nordenskioldii along with Chetoceras contortum and 
C. debile made up most of the diatom maximum. During the next few 
years the numbers of Thalassiosira were much lower, but it again appeared 
in quantity in 1912 (6 millions on April 29th) and 1913 (64 millions on 
