THE MICROPLANKTON OF PLYMOUTH SOUND. 145 
(14) Coscinodiscus excentricus Khr. Common from September to 
May. 
(15) C. radiatus Ehr. Common from September to May. C. excentricus 
and C. radiatus are the most abundant species. 
(16) C. sub-bulliens Jérg. Only noticed from September to December, 
Not very common. 
(17) C. Granw Gough. Begins in November and remains till April. 
Sometimes common in December, January and February. 
(18) Actinocyclus Ehrenberg: Ralfs. In tow nettings only. Rare. Sep 
tember. 
(19) Actinoptychus undulatus (Bail.). From the middle of September 
to the end of April, never very abundant, more frequent in 
tow nettings than in the water samples. Not seen at all in the 
summer. 
Genus RHIzOSOLENIA (Khr.) Brightw. 
With the exception of R. robusta which is the only winter 
form all the species of Rhizosolenia are markedly summer 
forms; beginning to be abundant in May they continue 
common until the end of September at all depths. If we 
compare this with the Port Erin records .we find it agrees 
well except for the fact that at Port Erin there are very few 
present in August. 
The curve (Fig. 4), giving the distribution of the various species, 
shows R. Stolterfothii as much the most abundant with a big 
maximum in June. As mentioned above, however, R. Shrubsoler 
occurred in enormous numbers in June in the tow nets, of a large 
size, and was not adequately represented in the water samples. 
The maximum of the species on the curve ought to rise very much 
higher. I find that R. Shrubsole: and Stolterfothi run together to 
a great extent, although Shrubsolec almost disappears in July, 
whilst Stolterfothic continues common well into September. The 
genus is hardly represented at all from November to April. Its 
absence being very striking, &. alata follows R. Stolterfothi closely, 
although it is not so common. &. hebetata form semispina, has 
its maximum in May. AR. setigera is later, beginning in June and 
ending in September, with a maximum in August; thus it is 
later and remains less time than any of the others. All the species 
are abundant in the tow nets. 
