THE MICROPLANKTON OF PLYMOUTH SOUND. 143 
Large masses of a species of Mastigloia in a gelatinous sheath some- 
times occur at intervals and swell the number of diatoms largely. In 
these cases they are usually so numerous that I have estimated them in 
10 ce. instead of 50; I have also done this with other species when very 
numerous. 
Table IL shows the average number of diatoms in 1 cc. for each month. 
In the following details of the species the classification of ‘* Nordisches 
Plankton,” Vol. III, Gran (1905) is used. 
(1) Melosira Borreri Grey. Not common. In water samples, October 
to March. 
(2) Paralia suleata (Ehr.). Occurs almost all the year round in small 
numbers, but is essentially a winter species. Common from 
October to April with a maximum in November, then dwindles 
and picks up again in August. Nearly always goes through the 
nets.. More frequent at 5 and 7 fathoms although common some- 
times at the surface. Belongs, properly speaking, to the bottom 
but very often comes up to be a true member of the plankton. 
(3) Skeletonema costatum (Grey.). Very common for nearly the whole 
year, but has periods of disappearance. Rare in December and 
part of January, June and July. Maximum of 250 per ce. in April, 
when it helps largely in making the spring diatom maximum. Very 
numerous in August, September and October. A smaller second 
maximum in August, and in October, 1915, a still smaller one. 
Lohmann considers Skeletonema costatum the most important diatom 
at Kiel, where in June it reached a maximum of 780,000,000 per 
100 litres. He finds it prefers water of 10 m. depth. Gran (19126) 
shows it likes surface water, and I have found that although 
common in all three depths it is usually commonest in the surface 
samples and its maximum of 12,500 in 50 cc. is from the surface. 
This is one of the most important of the plankton diatoms at 
Plymouth, but passes through the net in quantity. 
(4) Thalassiosira gravida Cleve. This is the only species of the genus 
found commonly in the water samples. It is abundant from the 
end of March to the middle of September with an interval of 
scarcity in July and August. May and June are the months 
given by Herdman for the maximum of the genus at Port Erin, 
which agrees well with us. It occurs at all depths, but its maxi- 
mum in May of 316 in 50 ce. is from the surface. 
~ 
(5) T. Nordenskioldii Cleve. Not very common, occurring at intervals. 
Frequent in May. 
