154 THE DISTRIBUTION OF THE PLANKTON [PART II 



see that it fills up the North Sea, the Irish Sea, extends into the 

 Skagerak and forms a narrow edging along the Norwegian coasts. 

 In Cleve's chart the Tripos type also includes those forms of 

 plankton which are characteristic of the shallow seas. 



Just as a number of oceanic types of plankton can be recognised 

 so types of plankton which are characteristic of the shallower seas 

 can also be distinguished. By shallow seas I do not mean the 

 restricted and highly variable inshore waters within 10 to 20 miles 

 from the land but the wider expanses of water such as the North 

 Sea, the central parts of the Irish Sea and St George's Channel 

 and so on. Here we find a more uniform plankton than that which 

 is to be seen inshore, and which can easily be divided into several 

 main types. Thus Cleve recognises the following shallow water 

 or neritic plankton types : 



Didymus-plankton, a category which includes a number of 

 diatom and peridinian forms, and which is named from the diatom 

 Chaetoceros didymus, an abundant species. It is a southern North 

 Sea type. 



Northem-Neritic-Plankton. The diatoms Leptocylindricus 

 danicus, Skeletonema costatiim, Chaetoceros laciniosus, and Lauderia 

 annulata seem to be the most characteristic forms. It belongs to 

 the northern shallow water area. 



Arctic-Neritic-Plankton is characteristic of the shallow- water 

 Arctic seas. It is highly complex and its constituents cannot 

 easily be summarised. 



Concinnus-Plankton is a characteristic North and Irish Sea 

 plankton which appears in the early spring. It contains the 

 familiar diatom Coscinodiscus concinnus as its typical constituent. 



Halosphaera-Plankton. Now and then the shallow water 

 plankton all round the British seas contains the "unicellular 

 alga" Halospliaera viridis as its principal form. In the Irish 

 Sea it occurs chiefly in the early summer. 



Now the above names have been applied to certain plankton 

 collections because the latter contain one form which is usually 

 so abundant as to give the gatherings a certain individuality. 

 Associated with these there are, of course, a number of other 



