52 



species. During these two months the individual diatoms varied 

 greatlj^ in size. 



P. angulatum also appeared occasionally. 



Amphora. — This genus was represented almost entirely by 

 one species, namely, A. si:)ectahilis, which, like Pleurosigmay 

 occurred in small numbers all the year round, and showed a 

 sudden increase in April and May. A. arenicola was very rarely 

 present. 



Paralia. — P. sulcata was present in fair numbers throughout 

 the period, with the exception of July, when it was very rare, 

 and February when it w^as not recorded at all. A sudden rise 

 in the quantitj^ of this diatom took place in August, and it reached 

 a maximum number in January. The chains in this case were 

 usually much longer than those of Skeletonema. P. sulcata 

 appears to be present in this region all through the year without 

 any marked fluctuations in quantity. This agrees with Dr. 

 Lebour's account of the species as it occurs almost all the year 

 round at Pljanouth, but is essentially a winter species with a 

 maximum in November from which the numbers dwindle but 

 pick up again in August. 



Fragilaria. — F. striatula is another species w^hich is to be 

 found at Cullercoats at all seasons. It was almost as common 

 in the samples as Nitzschia closterium, but reached a maximum 

 height at a different time, viz., in July and August. During the 

 remaining months it occurred in fair numbers with great 

 regularity. At no time was it absent from the samples. It is 

 to this species, together with N. closterium, that the August 

 maximum of the diatom population is due. F. cylindrus also 

 occurs, but much less frequently. 



Fragilaria sp. is mentioned in the Plymouth list, but is marked 

 " rare " on every occasion. 



Coscinodiscus. — This genus was represented by C. radiatus 

 chiefly, together with C. exceniricus and C. lineatus in smaller 

 numbers. It was present throughout the year, but never in great 

 quantities. It appeared rather more frequently during March 

 and April. At Plymouth it is a winter form. 



Navicula. — N. fiisiformis, N. binodis and N. sphaeropliora, 

 together with small and unidentified species, occurred in small 



