53 



numbers during almost every month., and were rather more common 

 in Ma}^ 



The foUowmg diatoms are on the whole rare, but are inter- 

 esting in that they appeared in the samples only during certain 

 periods. 



Syredra pulchella, — Present from April to November. Rare, 

 except in April. 



Rhizosolenia shrubsolci, R. set ig era and R. calcar avis. — Present 

 from August to January. Very rare, but rather more in August. 

 The genus is represented by six species at Plymouth, v/ith a 

 maximum number in June. 



Chaetoceras sociaie and C. radians. — Occurred very rareh^ in 

 June, and rather more commonly in August. The genus is a 

 common one in the Pljnnouth records, and is represented by more 

 than sixteen species. 



Eucampia zoodiacus made its appearance suddenly in August, 

 when it was quite common, and remained until November. It 

 occurred only once more, in March. E. zoodiacus is present 

 occasionally at Plymouth from May to October. 



Asterionella bleaJcelevi occurred during three months, July, 

 August and October, and was fairly common in July. At 

 Plymouth it is recorded only twice, in November and December. 



Thalassiothrix curvata (with some T. frauenfeldii) was present 

 from May to November in fair numbers. 



Other diatoms occurring still more rarely or at irregular 

 intervals are included in the general list of species given in Table I. 

 A few still remain unidentified. 



Two of the species recorded, namely, Surirella spiralis and 

 Cymbella cymbiformis are fresh water forms. 



PERIDINIALES. 



Members of this group appear in the tank samples in numbers 

 which give a curve very similar to the Plymouth one, the greatest 

 numbers being present in July and fSeptember here, and in May 



