INVESTIGATION OF THE PLANKTON OF THE IRISH SEA. 157 
those that are best for diatoms; but in 1912 both groups were especially 
strong. 
The chief genera represented in our plankton are Ceratium (chiefly 
C. Tripos) and Peridinium *—several species of each,— and as a rule 
Ceratium is the more abundant and the earlier in attaining high numbers ; 
but in 1912 Peridinium reached far greater quantities than Ceratium. The 
maximum was on May 9th when one haul gave 8,650,000, the greatest 
number of Peridinium we have ever recorded during these investigations. 
Peridinium is to be regarded as an oceanic form, and this exceptional 
abundanee in 1912 agrees with other evidence f that that year our western 
coasts showed an unusually large invasion of Atlantie organisms. 
In 1914 there were two well-marked dinoflagellate maxima, an earlier in 
May and an exceptionally late ono in November. 
Ceratium Tripos shows records of from 300,000 to 600,000 per haul in 
July 1913, June 1915, July 1916, 718, 19, and June 1920, We may take 
as a final example of the numbers of this group the year 1921. The early 
summer proved favourable for dinoflagellates, with a maximum in late May 
and June. By March 24th (unusually early) Ceratium Tripos had reached 
18,000 per haul, and in early April all the common dinoflagellates were in 
the thousands. On May 13th C. Tripos reached 40,000, and on June 13th 
and 16th about 100,000. Species of Peridinium at the same time in the 
middle of June reached 170,000, but had had an unusually early maximum 
of 300,000 on May 20th. 
Ceratium Tripos, although sometimes regarded as an oceanic form, is pre- 
sent all the year round in the Irish Sea, and in some years (e. g., 1918, ’19, 
>20, &e.) the average of all hauls amounts to several thousands in every 
month, and during June to November runs into tens of thousands. 
Two other species of Ceratium, C. furca and C. fusus, are also commonly 
present in Irish Sea plankton, and in the summer of 1921 C. furca was 
especially abundant at Port Erin and was in an unusually active condition, 
several specimens at once being commonly seen moving across the field of 
view when the freshly caught plankton was put under the microscope. 
COPEPODA. 
Next after the diatoms, the Copepoda are the most important group in the 
plankton of the Irish Sea from the point of view of economics and meta- 
bolism. The Copepoda, as a whole, are a summer and autumn group, and 
* There has been some confusion between allied species in the records from our seas, so 
it is best to deal with them as a generic group. 
+ See Herdman and Riddell on “ Plankton of the West Coast of Scotland,” Trans, Biol. 
Soe. L’pool, vol. xxvii, 1913. . 
LINN. JOURN.—BOTANY, VOL. XLVI. N 
