OF DIATOMS AND COPEPODA IN THE IRISH SEA. 119 
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 
Chetoceras and Thalassiosira, 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, Chetoceras debile in late April, 
Chetoceras sociale in May, Chetoceras teres and Rhizosolenia Shrubsolei in 
early June, and Rhizosolenia (several species) and Guinardia in later June. 
The autumn Diatom maximum is constituted mainly in the Irish Sea by 
Chetoceras 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 they may 
occasionally be carried in shore and so form a part ef the coastal plankton ; 
while other species are “neritic,” having their origin aad their home in 
coastal waters and not being found normally in the open ocean. For 
example, Chetoceras densum, C. boreale,- Coscinodiscus radiatus, and Rhizoso- 
lenia. semispina are supposed to be oceanic; while Biddulphia mobiliensis, 
Chetoceras subtile (and other species), Coscinodiscus concinnus, Rhizosolenia 
setigera, and Lauderia borealis are probably neritie. 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. 
