202 PROF. 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 water 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 faetors, such as 
the weather (temperature, sunshine, rainfall, wind, ete.) at the time, and 
previously. 
Of the six Copepoda discussed above only one— Temora—is a neritie form ; 
the others are all usually regarded as oceanie, that is as having their true 
home and centre of distribution somewhere to the north, west, or seuth in 
the open Atlantic. 
The following list gives an approximate indication of what is supposed 
to be the source of these five oceanie Copepoda :— 
Calarus.—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 (O.) for oceanic, and a (?) indicates that the evidence seems to 
me conflicting or inconclusive *:— 
Biddulphia mobiliensis (N.), B. sinensis (O.). 
Coscinodiscus radiatus (O. ?), C. concinnus (N.), C. Grani (N.). 
Lauderia borealis (N.). 
Chætoceras boreale (O.), C. eriophilum (O.), C. decipiens (O ?), C. densum 
(0.), C. eontortum (N.), C. debile (N.), C. diadema (N.), C. sociale (N.), 
C. subtile (N.), C. teres (N.). 
1 halassiosira gravida (N.), T. Nordenskioldi (N.). 
Rhizosolenia alata (O.), R. semispina (O.), R. setigera (N.), It. Shrubsolei 
(N. ?). R. Stolterfothi (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, 
