INVESTIGATION OF THE PLANKTON OF THE IRISH SEA 155 
May 16th), and in April 1915 and 1917 it was again abundant at Port Erin. 
In 1918, ’19, 720, and 721 the maximum was early in May, but the 
numbers not high. In fact, it has only appeared in quantity in our gatherings 
in April and May, and not always then. 
A few other prominent genera of diatoms are worthy of notice for one 
reason or another. Coscinodiscus is an early spring or even winter form, 
appearing along with Diddulphia while the water is still at its coldest and 
forming the first increase in the phytoplankton, generally in March but 
sometimes as early as February or even January. It is only very rarely 
that either Coseinodiseus or Biddulphia runs to millions per haul in our 
plankton, nor do they ever show the almost miraculously sudden increases 
that we see in the case of Chefoceras and Rhizosolenia; but they are 
individually large diatoms, and consequently “bulk large” in appearance in 
a plankton sample under the microscope. Though they occur at the same 
time of year they by no means flourish equally in the same year. For 
example, in 1912 the numbers for Biddulphia were considerably lower than 
those in 1911, and, on the other hand, the numbers for Coscinodiscus were 
higher in 1912 than in 1911. In 1919 Biddulphia attained to higher 
numbers in November than at the spring maximum—an exceptional 
occurrence. 
Since 1909 two species or “forms” of Biddulphia have occurred commonly 
in the Irish Sea plankton—the commoner British Biddulphia mobiliensis 
(or B. regia, or “forma regia ” of some) and the rarer, possibly exotic, 
species or form “ sinensis." In my former paper * I discussed the curious 
history given by Ostenfeld of the spread of B. sinensis through the seas of 
North-West Europe since its appearance at the mouth of the Elbe in 1893; 
and in our Sixth Annual Reportt we gave a plate showing variation in 
Biddulphia, from a series of photo-mierographs by Mr. A. Scott, which I 
repeat here in order to add the comments which Prof. Ostenfeld has sent to 
me ina letter. As recorded in our previous report, in 1911 and 1912 we 
noticed some specimens of B. sinensis which showed the normal characters 
of that form at one end of the cell, while the other end had the appearance 
of B. regia. The structure of these abnormal forms, which have continued 
to appear from time to time, is shown by figs. 5, 7, 8, 9, 10, 12, 14, and 15 
on the Plate (Pl. 7). All these figures show clearly that one end of the cell is 
Biddulphia sinensis, while the other end shows a decided approach to the 
appearance of B. regia or mobiliensis. 
We took the view in our 1913 report that the “ sinensis" form was not a 
distinct species, but a mutation of B. mobiliensis (or regia). Since then 
Prof. C. H. Ostenfeld, after examining a sample, has written to me as 
* “Spolia Runiana,” III. 1918-—/oe. cit. p. 179. 
+ Trans, Biol. Soc. L'pool, vol. xxvii. (1913) p. 204. 
