124 1'ROF. W. A. HERDMAN ON THE DISTRIBUTION 



by bodies of warmer and Salter Atlantic water (re-inforced possibly by por- 

 tions of a deeper outflowing Mediterranean current) carrying in oceanic 

 plankton, and more rarely perhaps by Norwegian or Arctic waler causing 

 an invasion of northern organisms. The variations which we find in different 

 years in the nature and amount of the plankton at the same localities no 

 doubt depend to some extent upon the volume and period of such southern or 

 northern invasions ; but they may depend also upon other factors, such as 

 the weather (temperature, sunshine, rainfall, wind, etc.) at the time, and 

 previously. 



Of the six Copepoda discussed above only one — Temora — is a neritic form; 

 the others are all usually regarded as oceanic, that is as having their true 

 home and centre of distribution somewhere to the north, west, or south in 

 the open Atlantic. 



The following list gives an approximate indication of what is supposed 

 to be the source of these five oceanic Copepoda : — 



Calavus. — N. Atlantic, about Iceland ("Boreal oceanic"). 



Paracalanus. — Southern, temperate and tropical Atlantic. 



Pseudocalanus. — N. Atlantic (" Boreal oceanic"). 



Oithona. — N. Atlantic ("Boreal oceanic"). 



Acartia. — N. Atlantic ("Northern styli-plankton "). 



Some no doubt live on during the year in the Irish Sea, but these indi- 

 genous populations are probably reinforced by waves of immigration from 

 outside. 



In the case of our Diatoms some of the species of a genus may be neritic 

 and others oceanic, as is shown in the following list, where (N.) stands for 

 neritic and (0.) for oceanic, and a (?) indicates that the evidence seems to 

 me conflicting or inconclusive *: — 



Biddulphia mobiliensis (N.), B. sinensis (0.). 



Coscinodiscus radiatus (0. ?), C. concinnus (N.), C. Grani (N.). 



Lauderia borealis (N.). 



Clut'toceras boreal e (0.), C. criopMlum (0.), C. decipiens (0 ?), C. densum 

 (0.), C. contortum (N .) , C. debileQ^.), G. diadema (N.), C. sociale (N.), 

 C. subtile (N.), C. teres (N.). 



1 halassiosira gravida (N.), T. Nordenshioldi (N.). 



Rldzosolenia alata (0.), R. semispina (0.), R. setigera (N.), R. Shrubsolei 

 (N.?) : R. StolterfotM (N. ?). 



Guinardia flaccida (N. ?). 



It is remarkable how small a number of forms make up the bulk of the 

 macro-plankton throughout the year. These half-dozen kinds of Diatoms 

 and half-dozen Copepoda are the all-important organisms upon which our 



* The matter was discussed more fully some years ago in a paper by Herdman and 

 Riddell in Trans. Biol. Soc. Liverpool, xxv. (1911) p. 178. 



