OF DIATOMS AND COPEPODA IN THE IRISH SEA. 99 



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, C/tcetoceras 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 

 Chcetoceras 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, Ciuctoceras, Coscinodiscus, Iihizosolenia, Thalassio- 

 sira, Guinardia, and Lauderia- In some of these genera (e.g., Guinardia 

 and Lauderia) as in the case of the Oopepoda 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., Chcetoceras, 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 apparent^ 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 |, while dealing mainly with the more minute 

 Protozoa and Protophyta which escape the tow-net and can oidy 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, Mav, and 

 June in the year when we took the most complete series of observations is from 3402 to 

 34 - 4 per mille. 



t Joura. Mar. Biol. Assoc, vol. xi. 2 (1917), p. 133. 



