KONGL. SV. VKT. AKADEMIENS HANDLINGAR. BAND 34. N:<» 2. II 



Period II. From Marek lV h to April 13 tJl . This period is a transitiona] one, as 

 both nothern and southern forras appeared in termingled. I consider the following species 

 to have been conveyed by not hem currents: 



Asterionella japonica. Chcvtoceros teres. 



Chcetoceros debil is. Skeletonema costatum. 



C. diadema. Thalassiosira gelatinosa. 



The following species are of southern origin: 



Cerataulina Bergonii. Guinarclia fiaccida. 



Ditylum Brightwellii. Rhizosolenia Stolterfothii. 



Eucampia zodiacus. 



These southern species appeared at Plymouth much låter, i. e. between the 24 th of 

 May and 19 th of July. 



Period III. From April 20 th to Jane 2 nä . During this period Phceocystis Pouchetii 

 appeared in enorir.ous abundance. Beside this species all others diminish,. just as at Plymouth, 

 where the same flagellate was predominant from the 4 th of April to the 12 th of May, thus 

 somewhat earlier than at Helder, so that there is reason for believing it arrived through 

 the Engl. Channel. 



Period IV. From June 9 th to August 25 th . The most characteristic feature in the 

 plankton are the considerable development of Noctiluca miliaris and the return of Gui- 

 nardia Jiaccida and Rhizosolenia Stolterfothii, driven away last period by the water con- 

 taining Flurocystis. Other southern forms, that appeared during this period are: Oithona 

 similis, Tintinnopsis campanula, Cyttarocylis serrata, Cluetoceros densus, C. didymus, C 

 Schiittii and RJdzosolenia Shrubsolei. 



This period corresponds to the period IV at Plymouth (24 VII to 18 VIII), although 

 the latter is characterized by the abandance of Rhizosolenia gracillima, not seen at Helder. 

 On the other hand Noctiluca occurred only sparingly at Plymouth. 



Period V. September l t}l to 28 th . During this period Biddulphia mobilensis and 

 Coscinodiscus concinnus appear, and in their company boreal forms, such as Ceratium longipes, 

 ( Ixctoceros debilis and Phceocystis. Also new southern species arrive, as Pyrophacus horo- 

 logium, Bacteriastrum varians and Lithodesmium undulatum. 



This period evidently corresponds to the periods V and VI at Plymouth. 



