OF DIATOMS AND COPEPODA IN THE IRISH SEA. 101 



The maximum on the Diatom curve ranges from March to May. In 1907 

 it was in March, in 1908 in May, and in 1909 in April. In some years the 

 Diatom maximum may be divided into two parts, an earlier due mainly to 

 Chcetoceras and Tlialassiosira, and a later in June due to Rhizosolenia and 

 Guinardia. A common order of succession for the species which contribute 

 most largely to the Diatom maxima is — Biddulphia mobiliensis and 

 Coscinodiscus radiatus in early April, Chcetoceras debile in late April, 

 Chcetoceras sociale in May, Chcetoceras teres and Rhizosolenia Shrid>solei in 

 early June, and Rhizosolenia (several species) and Guinardia in later June. 

 The autumn Diatom maximum is constituted mainly in the Irish Sea by 

 Chcetoceras subtile and Rhizosolenia semispina. Certain species of most of the 

 genera we are dealing with are commonly regarded as ; ' oceanic " in the 

 sense that they are characteristic of the open sea, although the}' may 

 occasionally be carried in shore and so form a part of the coastal plankton ; 

 while other species are " neritic," having their origin and their home in 

 coastal waters and not being found normally in the open ocean. For 

 example, Chcetoceras densum, C. boreale, Coscinodiscus radiatus, and Rhizoso- 

 lenia semispina are supposed to be oceanic ; while Biddulphia mobiliensis, 

 Chcetoceras subtile (and other species), Coscinodiscus concinnus, Rhizosolenia 

 setigera, and Lauderia borealis are probably neritic. Other species of these 

 genera are of doubtful position in this classification, or it may be are 

 " panthalassic " or equally at home in both regions. 



Biddulphia. 



This is a winter and early spring group of Diatoms, generally appearing 

 in September or October, becoming more abundant in November and 

 reaching a maximum in March or April. It dies off during May, and is 

 practically absent in June, July, and August. 



The species, or forms, that occur in our plankton are Biddulphia mobiliensis 

 (? B. regia) and B. sinensis (fig. 3). This is one of the cases where there is 

 some difference of opinion as to the validity of species. Whether B. regia 

 is only a form of B. mobiliensis, and what is the exact relation of B. sinensis 

 to the others, has been and may still be a matter of discussion. Specimens 

 can be picked out that seem distinct and characteristic, but others occur in 

 nature that are intermediate and possibly abnormal *. 



B. sinensis is an exotic, oceanic form which, according to Ostenfeld, made 

 its appearance at the mouth of the Elbe in 1893, and spread during successive 

 years in several directions. It appeared suddenly in our plankton gatherings 

 at Port Erin in November 1909, and has been present in abundance each 

 year since. Ostenfeld, in 1908, when tracing its spread in the North Sea, 

 predicted that it would soon be found in the English Channel. Miss Lebour, 

 who has recently examined the old plankton gatherings at the Plymouth 



* See forms figured in Trans. Biol. Soc. Liverpool, xxvii. (1913) p. 210. 



