THE MICROPLANKTON OF PLYMOUTH SOUND. 149 
(42) OC. debile Cleve. Not very common, May and June. 
(43) Cheetoceras spp. Species which could not be identified were common 
in July and August. 
(44) Eucampia zoodiacus Ehr. Occasionally from May to October. 
(45) Streptotheca thamensis Shrubs. Common from September to April, 
otherwise rarely seen. More frequent in tow nettings than in 
water samples. 
(46) Cerataulina Bergont H. Perag. Fairly common in May and June. 
Genus BrpDULPHIA Gray. 
The Biddulphia species are practically confined to the autumn, 
winter and early spring, being almost entirely absent in the 
summer. This agrees fairly well with the Port Erin records, 
although there, in small numbers only, they are found in the 
summer. At any rate they may be regarded as winter, or early 
spring, and autumn forms. B. mobiliensis, regia and sinensis arc 
all common in the early spring, winter and autumn. Whether 
B. regia and sinensis should be regarded as good species is a 
matter discussed at length by Herdman (1912), who has shown 
that intermediate forms are to be found and has figured forms 
from Port Erin which appear to be half B.-senensis and _ half 
B. regia or mobiliensis, his final decision being that they are 
probably all the same species. He therefore regards B. sinensis 
and B. regia as distinct forms of B. mobiliensis. There seems to be 
no doubt about the sudden appearance of the exotic species 
B. sinensis in numbers at Port Erin in November, 1909, and 
also that it suddenly appeared at the mouth of the Elbe in 1903, 
as 1s shown by Ostenfeld (1908) : having spread from the mouth 
of the Elbe into various places including the North-EHast of 
Scotland it was then found on the Belgian coast, Ostenfeld 
accounting for its presence there by imagining a reversal of 
the usual north-going current. Its first appearance at the 
mouth of the Elbe Ostenfeld thinks is probably due to its being 
taken there by some ship. In 1908 he predicted its discovery 
in the Channel, as up to that time it had not been found 
to occur there. In order to ascertain whether it was present 
in Plymouth in former years (it certainly is common here now) 
I examined a large number of old tow nettings mostly from 
the West Channel, Plymouth, and all from this district. Beginning: 
NEW SERIES.—VOL. XI. NO. 2. MAY, 1917. L 
