8 P. T. CLEVE, THE PLANKTON OF THE NORTH SEA, ENGLISH CHANNEL, ETC. 



Evadne Nordmannii. C. Schuttii. 



Paracalanus parvus. Ditylum Brightwellii. 



Parapontella brevicornis. 



These southern forms cannot thus be assumed to have come in the company of 

 such forras as charaeterize the period III., and it seems probable that they were swept 

 down from the British coast by the northern flows. 



Period III. May 24 th to July 19 th . The arctic speeies had disappeared, almost 

 completely, hut beearae replaced by southern forms, e. g. Guinardia flaceida, which 

 appeared already before this period, but decreased during the period II. Among such 

 forms we note: 



Acartia Clausii. Chcetoceros densus. 



Centropages typicus. Guinardia flaceida. 



Oithona similis. Rhizosolenia Shrubsolei. 



Paracalanus parvus. 



And, besides, the following new ones: Isias clavipes, Cerätium fusus and Rhizo- 

 solenia Stolterfothii. 



It seems ra ost probable that these forms came with water from the coast banks of 

 the temperate Atlantic. 



Period IV. July 24 tJl to August 18 th . Characteristic speeies is Rhizosolenia 

 gracillima. In its company arrived a number of southern neritic speeies, among which 

 some continued during the next period. Such forras were: 



Coryceeus anglicus. C. didymus. 



Euterpe acutifrons. C. Schuttii. 



Cyttarocylis serratus.- Ditylum Brightwellii. 



Tintinnopsis campanula. Rhizosolenia corpulenta. 



Chcetoceros curvisetus. 



Several of these speeies are the sarae as appeared in the spring together with 

 boreal forms. I presume therefore that the specirnens in the period IV represent a fresh 

 set, that arrived from the south, probably from the Bay of Biscay or the French coasts. 

 The rare occurrence of Peridinium exiguum and of Dinophysis homunculus corroborates 

 such an opinion. 



Period V. August 24 th — 31 st . This short period is remarkable for the sudden 

 reappearance of boreal forms as Asterionella japonica, Cha>toceros debilis and Skeletonema 

 costatum. Together with them also Streptotheca thamesis appeared, but sparingly. They 

 seem to be the first signs of the presence of the boreal flows, that charaeterize the next 

 period. 



Period VI. September 10 th to December 28 th . The raost characteristic speeies is 

 Coscinodiscus concinnus. In its company a number of boreal forms appeared viz.: 



Fritillaria, borealis. Chcetoceros decipiens. 



Onccea minuta. Coscinodiscus oculus indis. 



Peridinium depressum. Thalassiosira gravida. 



P. ovatum. 



