54 



and July at Plymouth. The increase takes place with great 

 suddenness at both places. (Fig. I.) 



Here again, as in the case of the diatoms, we have to record 

 a much larger number of individuals, namely, 5,000 per 50 cc. 

 at the maximum, as against 1,000 per 1,000 cc. at Ptymouth, 

 and, again, I consider that the number in our samples is abnormal. 

 This feature is noticeable during the whole period, and would seem 

 to be due to the numbers of Peridinians remaining in the tanks 

 between successive j)umpings and reproducing repeatedly in the 

 confined space. Their quantity, too, is not depleted by their 

 forming food for higher organisms, so that, together with the 

 numbers regularly pumped in from the sea, this gives us an extra- 

 ordinarily high proportion of these forms in the tanks. 



From July to September the numbers ranged from 1,000 to 

 5,000 per 50 cc. In February and April they touched the 500 

 line, and rose to 900 in May, but during the remainder of the 

 time the numbers of Peridiniales present w^as never above 200 

 per 50 cc. Even when least plentiful the numbers are consider- 

 ably in advance of those given b}^ Dr. Lebour for the corresponding 

 months. From her curve we find that at no time between 

 October and May do these forms reach the 100 line per 1,000 cc. 

 Evidently the tanks are alwaj^s a most unreliable source of supply 

 for the estimation of the Dinoflagellate population of the sea. 



An anah^sis of the records shows the commonest species of 

 Peridiniales in the tanks to be Peridinium orbicidare and P. 

 thorianum, which occur in greater or smaller numbers all the 

 year round. The former appears with greatest frequency from 

 September to January, and the latter in April, May, July and 

 September. P. pellucidum is common between September and 

 January, and appears in small quantities during most of the 

 remaining months. It is by these three species chiefly that the 

 September maximum is caused. 



Other species which occur fairly regularlv are Glenodinium 

 gymnodinium, Spirodinium fissum and Amphidinium operculatum. 

 Further details of the occurrence of the last-named species are 

 given in another paper (page 57). A. crassum and A. rotundaium 

 also occurred less frequently. 



The remainder of the species of Peridiniales recorded appeared 

 in and disappeared from the samples spasmodically. 



