OF DIATOMS AND COPEPODA IN THE IRISH SEA. 147 
When we turn to the Diatoms the case is rather different. There several 
of the more prominent genera are represented by a number of common 
species, and moreover some of the species are closely related, and variable, 
so that doubts may arise as to the exact identifications, and authorities may 
differ as to the relative proportions in which certain species or varieties of, 
say, Chetoceras or Biddulphia are present in the plankton. Under these 
circumstances I have considered it will best serve my purpose, which is a 
general and economic rather than a detailed speciographic one, if I deal with 
generic titles only, grouping together for example all the species of 
Chetoceras that may occur under that one name. I have chosen the following 
genera as being the most important representatives of the Diatoms in our 
plankton :—Biddulphia, Chetoceras, Coscinodiscus, Rhizosolenia, Thalassio- 
sira, Guinardia, and Lauderia. In some of these genera (e.g., (ruinardia 
and Lauderia) as in the case of the Copepoda there is only one possible 
species in question, in Biddulphia it is in most cases only the species B. mobi- 
liensis, but in others (e. g., Chetoceras, Coscinodiscus, and Rhizosolenia) there 
are usually several allied species occurring together in profusion in any large 
gathering of the genus. 
I may add that our commonest species in the Irish Sea off Port Erin are 
not necessarily those that are most abundant in other seas of North-West 
Europe. For example, in the Baltic near Kiel, according to Lohmann, the 
most abundant Diatom is Skeletonema costatum, a comparatively rare form in 
our plankton, and George Murray similarly found that to be the commonest 
form he had met with in a plankton survey of some of the more sheltered 
lochs of the West of Scotland. It is, however, one of the minuter forms 
which readily escapes notice, and may to a considerable extent pass through 
the meshes of the net. 
Then again, in July 1911, in Upper Loch Torridon, on the West Coast of 
Scotland, I got a haul of 334,000,000 Nitzschia delicatissima, which is rare 
with us in the Irish Sea, but is apparently more abundant at Plymouth. 
I think it probable, however, that our Port Erin results will be found to hold 
good for the more open sea-water of high salinity * around the British Isles. A 
valuable paper which appeared recently on the Plankton of Plymouth Sound, 
by Dr. Marie V. Lebour t, while dealing mainly with the more minute 
Protozoa and Protophyta which escape the tow-net and can only be obtained 
by centrifuging samples of water, gave also some records of the occurrence 
of some of the larger forms which enables a comparison to be made between 
the plankton conditions in the English Channel and in the northern part of 
the Irish Sea. 
* The salinity off Port Erin averages about 34:2 per mille. Its range for April, May, and 
June in the year when we took the most complete series of observations is from 34:02 to 
34:4 per mille. 
+ Journ. Mar. Biol, Assoc. vol. xi. 2 (1917), p. 138. 
