KONGL. SV. VET. AKADEMIENS HANDLINGAR. HAND 34. N:o 2. 7 



VI. Plymouth in 1899. 



The soasonal changes in the plankton will be examined for certain periods witli 

 reference to the prevailing plankton types. 



Period I. January 17 th to Marek 14 fh (30 fh ). Characteristic species are Biddulphia 

 mobilensis, Halosphcera viridis, Coscinodiscus concinnus and C. fxcentricus. Besides these 

 forms, which probably derive from the northern coasts of the British islands and from 

 Scotland, there occurred a number of both boreal and sonthern forms. 



Among' the northern forms we note: 



Fritillaria borealis. Chwtoceros teres. 



Onccea minuta. Thalassiosira gelatinosa. 



Chcetoceros decipiens. 



These forms do not continue during the next period or occurred then sparingly 

 only. The following northern species continue for the next period: Pseudocala?ius elon- 

 gatus, Peridinium ovatum and Thalassiosira gravida. 



Among southern forms, besides such as remained for the whole year, we note: 



Centropages typicus. Parapontella brevicornis. 



Corycceus anglicus. Chcetoceros Schiittii. 



Euterpe acutifrous Corethron hystrix. 



Onccea subtilis. Ditylum Brightwellii. 



Paracalanus parvus. Rhizosolenia robusta (rr). 



Most of these species appeared towards the end of February, or låter, and con- 

 tinued during the following period. 



Period II. April 4 th to May 12 th . This period is remarkable for the abundance 

 of the aretie Phceocystis Pouchetii. In its company a number of other aretie or boreal 

 species appeared, most of which did not survive during the next period. Among these 

 boreal forms we note: 



Pseudocalanus elongatus. Chcetoceros f urcellatus. 



Temora longicornis. Leptocy lindrus danicus. 



Peridinium ovatum. Skeletonema costatum. 



P. pellucidum. Thalassiosira gravida. 



Asterionella japonica. T. Nordenskiöldii. 



Chcetoceros debilis. 



Calanus jinmarchicus is the only boreal specied that survived this period. 



The flows of boreal water, that commenced during the period I., continued to the 

 middle of March, and took during this period a more decided aretie character. 



During the period II. also a certain number of southern forms occurred, but 

 among them several seemed to die out or to be driven away by the water containing 

 Phceocystis. Such forms were: 



Corycceus anglicus. Choztoceros curvisetus. 



Euterpe acutifrons. C. didymus. 



