51 



An analysis of this table gives the following particulars of 

 these genera : — 



Nitzschia. — This genus is represented chiefly by N. closierium 

 and a few N. seriata at odd times. N. delicatissima appeared 

 very rarelj^ N. closterium is present throughout the year, but 

 is very scarce in January and February. It attains its maximum 

 in April, Avhen numbers of the short form with straight ends occur 

 amongst the ordinary long form. {See " j). 152.) This is the 

 commonest diatom in the samples, and shows a striking differ- 

 ence from the Plymouth records, where it occurs throughout the 

 year, but never in large numbers unless entangled in masses of 

 other diatoms. Here it was very rarely found entangled, and 

 the individuals w^ere in the great majority of cases entirely 

 separated. Occasionally two were found together but never 

 more. 



Skeletonema. — S. costatum was present all the year, except 

 from November to February inclusive. A great increase in its 

 numbers occurred suddenly in April and May, and it is to this 

 species, together with Nitzschia closterium, that the spring 

 maximum is due. The chains of 8. costatum w^ere very short, 

 the majority containing only four to six cells. A fair number 

 were, however, ten to twelve cells long. 



S. costatum is recorded as being very plentiful at Plymouth, 

 and as appearing in enormous numbers at Kiel. It has periods 

 of total disappearance at these places also. 



Melosira. — Two species of Melosira appeared in the samples 

 M. jurgensii being the chief one, whilst M. nuynmuloides occurred 

 at rare intervals. At no time was this diatom common, and it 

 was absent from November to February and in x4pril. It reached 

 its maximum in May and June. 



Pleurosigma. — P. acutum was one of the larger species which 

 was evidently able to find its way into the tanks easily, as it 

 occurred practically all the year round. From June to February 

 it was represented only very poorly by single specimens here and 

 there, though at no time was it entirely absent, except during 

 January. In March there was a slight increase in quantity; and 

 in April and May a very considerable rise in numbers of this 



