142 ; MARIE V. LEBOUR. 
early and occurs in August. It is very nearly as big as the spring maxi- 
mum. Also in the curve there is another maximum in October, 1915, after 
which the numbers are very low, until they suddenly rise enormously 
in April. The October maximum is possibly the ordinary autumn 
maximum occurring later in 1915 than in 1916. For the rise in April 
Skeletonema costatum is almost wholly responsible ; in May Chetoceras 
species are mainly responsible, together with Nutzschia delicatissima, 
Thalassvosira gravida and helped by Rhizosolenia species and Lauderia 
borealis. For the August maximum Chetoceras again is to the fore with 
Asterionella japonica, Mastigloia at times in numbers, Rhizosolenia species 
and Nitzschia species. The rise in October, 1915, is due to Mastigloia, 
Cheetoceras, Lithodesmium undulatum and Skeletonema costatum. 
The diatoms of the first or spring and summer group include the genera 
Asterionella, Chetoceras, Lauderia, Nitzschia, Rhizosolenia and Thalassio- 
sira; those of the autumn and winter group include Biddulphia, Coscino- 
discus, Paralia, Streptotheca and Thalassiothrix. One of the most important 
diatoms is Skeletonema costatum, which, although occurring practically 
all the year round, yet has certain times of total disappearance for short 
periods. It cannot be placed in either of the above-mentioned groups 
as it extends over both. 
We find this year the genera Biddulphia and Coscinodiscus disappear 
suddenly and do not continue in small numbers through the summer, as 
is the case generally at Port Erin. Gough, however, has recorded Bid- 
dulphia mobiliensis in June and August from Plymouth, so it must 
occasionally be present ; also Coscinodiscus species very rarely. Paralia 
and Thalassiothriz are essentially winter forms here, the latter stopping 
abruptly in the sprmg and the former being much commoner in the 
winter, although occurring throughout the year. The records of Bygrave 
and Gough are here also in agreement. 
Several important species have only one maximum in the year. Monthly 
curves show a gradual decrease from it. Asterionella japonica (July), 
Rhizosolenia Stolterfothi (May), R. alata (June), R. Shrubsoler (May), 
Rh. hebetata f. semaspina (May), R. setigera (August), are examples ; also 
Biddulphia species and Coscinodiscus species (autumn to spring) the curves 
of which could not be exactly determined because of their presence only 
sparingly in the water samples. The following are some of the most 
important species which have two maxima: the larger in April, May or 
June, usually very much exceeding the second in August or September : 
Skeletonema costatum (April and September), Chetoceras curvisetum (May 
and September), Lauderia borealis (April and August), Thalassvosira 
gravida (May and September). These results agree roughly very well 
with the previous records for Plymouth by Gough and Bygrave. 
