OF DIATOMS AND COPEPODA IN THE IRISH SEA. 



103 



(1911, 1914, and 1915) in our ten years the November monthly average is 

 the highest in its year. Moreover, two of these November records, viz., 

 341.231 in 1911 and 406,100 in 1914, are the highest in the whole series. 

 So Biddulphia may be regarded as characteristic of the late autumn 

 (November) as well as the early spring plankton. 



Gosoinodiscus. 



The more abundant species that our records deal with are Coscinodiscus 

 ooncinnus, C. Grani, and C. radiatus. They are mainly winter and spring 

 forms, the maximum of the genus occurring always in our experience in 

 either March or April. Coscinodiscus (fig. 4) agrees very closely with 

 Biddulphia in its distribution throughout the year, beginning in late autumn, 

 maintaining its position throughout the winter, increasing in January or 



Fig. 4. — Photo-micrograph of an early spring phytoplankton 

 consisting mainly of Coscinodiscus and Biddulphia. 



February, and then more rapidly in March and April, and dying away before 

 the height of the summer; but it sometimes continues to be present in small 

 quantities further into the summer months than is the case with Biddulphia. 



The presence of Coscinodiscus in quantity in the spring plankton is easily 

 seen through a glass jar with the unaided eye, as the little rounded drum- 

 like cells give a characteristic granular appearance to the gathering. 



Our largest records of Coscinodiscus are : — 



From four to five hundred thousand per haul on several occasions 

 between middle of March and middle of April, 1911, 1912, 1913, 1915, 

 and 1916. 



About nine hundred thousand on several occasions at end of April 1914. 



From one million to close on five millions on two occasions in middle of 

 April 1915. 



So Coscinodiscus may outrival its companion form Biddulphia at the time 

 of the vernal maximum, but does not attain to such high numbers in late 



