THE MICROPLANKTON OF PLYMOUTH SOUND. 139 
species which do not agree; for instance, Coscinodiscus Grani has a 
maximum at Kiel in August, whereas I found it confined to the period 
from November to April, when it is fairly evenly distributed. The 
maximum of Prorocentrum micans in August or September seems to be 
well established. Ostenfeld (1913) is here also in agreement. Ceratiwin 
fusus also has its maximum at this time, and Pyrocystis lunula, which 
at Plymouth is only recorded in these months. However, I find that in 
many cases species having a spring maximum at Kiel have it here in 
the summer. 
A comparison of the present results, with those of Gran (1912) is difficult 
as his are only for the month of May and from so many stations at 
various localities and many different depths. . However, if we take 
the Dutch results from the south-western part of the North Sea, 
which is the nearest to us of all the localities he makes use of, 
and compare them with the present records for the month of May 
only, we find the comparison is not without interest. Gran used 
the centrifuge entirely and the samples were all preserved. He usually 
took 50 cc. of the sample and calculated from it the number of individuals 
ina litre. Except in certain cases mentioned below, the numbers are 
not extremely different. Thus we find the species of Biddulphia present 
in very small numbers (only B. sinensis at Plymouth), a large number 
of several Chetoceras species in both (12 species with him, 8 with us). 
However, whereas there C. decipiens and debile are the prevailing forms 
(maximum numbers 11,500 and 6,500 per litre respectively) the prevailing 
forms here are C. curvisetum (maximum number 39,900 per litre) and 
C. pseudocrinitum (maximum number 30,000 per litre). A large number 
of resting spores of Chetoceras species are recorded by Gran and also 
by Lohmann. They were not recognised and therefore not recorded 
in the present paper. Lauderia borealis (Gran’s maximum 2,180 per 
htre) with us is more abundant (21,580 per litre). Paralia sulcata (Gran’s 
maximum 7,180 per litre at 30 m.), with us 700 per litre at 7 fathoms. 
Rhizosolenia species fairly abundant : 
Dutch records. Plymouth records, 
R. alata . : 72.800: (aes) 120 surface 
R. semispina . . 160 (20 m.) 500° 5.f. 
R. Shrubsoler . . 480 (10 m.) 1600: dif. 
R. Stolterfothir . . 8,360 (50 m.) 760 surface 
Thalassiosira gravida (Gran’s maximum 1,7€0 per litre, Plymouth 6,320 
per litre). Nitzschia delicatissima much more abundant at Plymouth, 
N utzschia closterium more abundant in the Dutch records. Of the Silico- 
